Love to see your math on that. (BTW, it's "amount".)
The other important figures of merit are: mission duration, repeatability, development cost, mission cost, etc.
For example, if a reusable lunar lander can carry 10 tons from the surface to lunar orbit every day, then your NEO mission better take less than 100 days, and that's pretty unlikely if you're planning on using SEP.
I was rounding off the numbers from the Keck report. It assumes 18.8 tons to LEO launched on an Atlas, to capture a "typical" 1300 ton NEO.
Lunar mining definitely has the advantage of being possible to scale up, but that requires a significant budget. Your lunar lander needs refueling, and even with oxidizer ISRU, the amount of mass you can put up into orbit is never going to be more than ~5 times the fuel mass you have to import from Earth. Asteroid mining gives a much larger mass multiplier than lunar mining with a chemical lander.
If you are considering using lunar hydrogen from polar craters, that implies a substantial scaleup of lunar activities. To begin with, it'll have to start with several exploration missions in a very challenging environment(no sunlight). So either you need the political will to develop a radioisotope rover to explore these craters, or you'll be limited to very short mission durations. While the rest of the moon allows for fairly easy prospecting, the interesting regions near the poles are notable more difficult. On the other hand, if you happened to have 250 tons of LOX already in lunar orbit from an asteroid capture, manned exploration missions would be quite a lot simpler...
Considering that small NEO retrieval is fairly near term and requires a smaller budget than an apollo-style moon landing, I see it as a logical stepping stone to a sustainable presence on the moon. An O'Neill style lunar mass driver would be a very nice thing to have but it requires landing large loads on the moon, and the cost of doing that could be greatly reduced with a prior infrastructure.
So you saying it takes a 20 ton spacecraft to get 1000 tons of rock to lunar orbit [or LEO- I don't think we want dusty rock on LEO, btw].
And you saying this require less spacecraft mass to lift 1000 tons of rock from the Moon.
No argument but I don't think it makes much sense to lift 1000 ton of lunar rock to Lunar orbit- unless it was a total of all the lunar samples which you intended to return to Earth.
But you saying one can get LOX from the rock. So a rock could have 40% mass being oxygen, but it seems one talking getting water from a carbonaceous asteroid and having this extracted water be the source of LOX.
So 20 ton spacecraft bring back 1000 ton rock and one can get 250 tons of water. Or 20 ton spacecraft brings 1000 tons worth of water ore. Which again is not what I would think is a good thing to ship off the Moon [lifting damp lunar soil].
So with the Moon, one goes to location at lunar surface which within 1/2 km distance would be, say 1 million tons of water ore.
One could ask how much is 1000 ton of water ore at lunar orbit [or LEO] worth vs million tons of water ore on lunar surface?
It seems one difference, is we seem to be going in direction of establishing property rights for space rocks, and not doing this in regard to the Moon. So loosely speaking one say the million tons of water ore is worthless without property rights- unless one explores the Moon and keeps the results of exploration a secret and sells it as information.
But anyhow what makes lunar water ore have value would be establishing of mining operation which one then may or may not have property rights in regard to it.
But dragging back a rock could have less problems in terms of who owns it, and therefore one could have the ability to sell it.
But question remains how much is the rock worth?
See, what the 20 ton spacecraft is doing, is not making water, but rather it's providing the rock a more favorable location- one still has mine it, which will require more than this 20 tons sent from Earth.
One could turn it around, and ask if someone had 1 billion or 10 billion to spend what kind of rock would they want to buy?
And they would want a rock in LEO for number of reason, and most obvious reason against that being it could a high cost liability if it damages other satellites. Another aspect is 100 tons of water or LOX is worth more at lunar orbit, than at LEO- assuming there is any demand for LOX or water near the Moon [going to Moon, or for use at GEO, or for going beyond Earth Moon system {ie, Mars}.
Or if what one mostly wants the 250 tons of water at Lunar orbit, how much does it cost to simply ship the 250 tons of water from Earth.
Or if one going to buy a rock in order to mine 250 tons water from the rock, spend more millions mining rock, how does it compare to shipping the 250 tons of water from Earth, instead?
Another aspect is one might *eventually* want 250 tons of water at lunar orbit, but you might not need much water in the nearest term. So maybe you want 50 tons in first couple of year then 100 tons [and 100 tons and more in the years after this]. So in terms of buying water ore, one want to buy some now, and have option to buy more later. Or one might only pay for water ore, once one has processed it into water- and so the whole purchase is future option [or how royalties are normally paid on Earth].
Because with water shipment from Earth, one also only pays when the water arrives and one can have as little water as one needs if shipped. Of course another option is simply shipping rocket fuel from Earth- or just LOX.
One advantage of shipping water is one is preparing for a business to process water from other sources other than from Earth. So your business model is to capture market for use of water for selling rocket fuel in space.
So you aren't going to mine water ore, you will buy water which has been distilled. So for that purpose, one might want only a small quantity of water shipped- say 5 tons. And you would demonstrate you can make rocket fuel from water in space. So with knowledge of someone getting rock which could be mine for water, one get 5 tons of water shipped from earth and you will show can use whatever water is mined in the future from the rock. But in any case, such a party is not going to pay more for water than the cost of it being shipped from Earth.
Now the advantage of mining water on the Moon is you can have more demand for water. Demand can include their stuff which can shipped off the Moon and send to Earth. People might want to come to the Moon and need water and need rocket fuel to leave the Moon. And largest demand could be exporting water and LOX to Cislunar space [same market available to the the space rock] and this includes any rocket fuel needed to get to the lunar surface.
Now anyone who wants to mine water on the Moon would start out as a buyer of rocket fuel at lunar orbit- so they could be buyers the water/rocket fuel made from space rock. And like the water buyer they don't care where it's coming from, they happy to buy rocket fuel shipped from earth if cheaper than from a rock. But they will want the amount they need when they need it.
And unless a lunar miner wanted a large operation [meaning there would be a large demand would could be met] I can't see them needed more than total 50 tons of rocket fuel at lunar orbit to land 50 tons of gear on the Moon. And unless the rocket fuel is hopeless cheap, once they start operation they would making their own rocket fuel, by shipping it to orbit.
And the need for the equipment to extract water could be far less than 50 tons. What could be more massive need is the power supply- landing acres of solar panels for use of splitting water and electrical power for other types of mining operations- though again depends expectation of amount market for electrical power.
If want some kind mass driver which would have a mass greater than say +50 tons, one would probably mine lunar iron for any need for the iron or steel for the mass driver [whatever type it is]. And one need iron for other mining operation including water mining. So bigger the scale, more likely it's more profitable to mine simpler components needed from the Moon.
But I generally think of lunar water mining starting out small scale, but if one is mining space rocks and providing cheap rocket fuel, this tend to drive lunar operations into larger scale. So tend to think in first few year one is only mining and making about 50 tons of rocket fuel per year, which within say 5 years is scaled up to 100 tons per year, and within decade or two 1000 of tons per year
And at 100 tons per year scale, one would have lunar water at about $500 per lb and LOX at $1000 per lb.
And shipped LOX in lunar orbit at about $3000 per lb. So if space rocks can from the start provide LOX at $3000 per lb, that be in range of it being hopeless cheap as far small scale lunar water mining at the beginning planning on competing with space rock water- though starting with out with large scale lunar mining may be a way to do it.
I tend to think only when there already a large market for water in high earth [or low earth] that one can profitable mine water for space rocks [one drag a 1 million ton rock into lunar orbit or Mars orbit]. And for small scale, I think PGM would better for space rocks. Though shielding for space stations and simply for use as gravitational masses could also have small scale market
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