I've always thought about doing this but I've been too lazy to crunch the numbers. And by "crunch the numbers" I mean assemble all of the vehicle prices in a list and have a calculator do it for me.
Anyways, let's start:
To buy every sea vehicle, you would have to spend $510,000. That's pretty reasonable, and that probably has something to do with the fact that there are only three of them in existence, one of them being completely free.
Next, every aerial vehicle would cost $14,500,000 - that's something of a step up from $510,000!
Finally, for the grand finale; to buy every land vehicle (purchasable with cash, of course) you would have to spend $48,250,000!
So, in total, you would have to spend $63,260,000 to buy every single purchasable vehicle in Mad City. But hey, that's not so bad! Here are your options (assuming you don't have V.I.P. and refuse to do any more than one single heist, of course):
Steal 632,600 bars of gold from the Cargo Ship (good thing it keeps coming back with more!)
Rob 21,087 Banks
Open 158,150 drawers in the Casino
Break 126,520 display boxes in the Jewelry Store
Raid 8,434 Pyramids (and evade an innumerous amount of lava-boulders in the process)
Empty the Nightclub's vault 10,544 times (sacrificing your E button in the process)
Encounter and completely empty a fully-loaded tram 7,882 times.
Good luck, and happy grinding!