That sounds excellent, though 50 billion is a bit too much. Say 10 billion?
You could send stuff to Jupiter's moons, to asteroids or to Mercury as well.
It would need to have some kind of a passive beacon to aid retrieval and prove that it is there. On Mars dust is a problem so a retroreflector might not hold up for long.
Ah, I falsely remembered hearing quotes on a sample return mission to be $20 billion, and was factoring in estimation errors and profit incentives.
Mars should be the first goal, I think. IANAP, but I think Jupiter's moons and Mercury are very substantially harder to collect samples from when compared to Mars. For Mercury it would be very difficult to escape the Sun's gravity well and get enough momentum to return to Earth orbit. Jupiter is also much further away than Mars and has a huge gravity well to escape from too. I'd wager that sample return missions are possible to them practically even given unlimited budget.
You could send stuff to Jupiter's moons, to asteroids or to Mercury as well.
It would need to have some kind of a passive beacon to aid retrieval and prove that it is there. On Mars dust is a problem so a retroreflector might not hold up for long.