All I can think of is being able to pay the mayor pardon a criminal for some absurd amount of money, sinking cash into criminal gangs, or paying off the mayor for whatever reason.
You just need to find something that rich players can sink their cash into to actually affect the game world, something they can keep doing instead of a one time buy vehicle (which should be a helicopter not a jet)
ohh i love the idea of linking it to nitramtj's spacebuild gamemode, that'd definitely be a first mod-wise. reminds me of a game i used to play called the universal, you'd go to planets and basically play a cityrpg like game, convert your money into space credits at a space port, and then be able to use your space credits to do various things in space. that's definitely a winner of an idea if you want to take the end game emphasis off crime and weapons.