Low probability, not incremental enough. I don't think you just pickup Dragon ECLSS and move it to a pressurised space in a much larger ship and have it work.
The first person to put boots on Mars will be famous forever. Perhaps there is a billionaire willing to pay for the priveledge? If a bidding war was started I wouldn't be surprised if SpaceX could get several billion for the first flight...
I think $500m is too low.