After I completed GTA IV I had around 600k just lying around, and that was AFTER I stocked up on ammo again. $600,000 just sitting in my pocket doing nothing.
Hopefully in GTA V we can buy cars. Perhaps there may be really rare cars which you can buy/import only which don't re-spawn. R* should make it so earning and saving money rewards you in the end. Not just with weapons, ammo, houses, clothes, etc. but with other things.
Giving a few $$$ to homeless people would be a cool idea too. Mundane, but why not?