There is an fast and a fun Method to get as much money as you like.

Other RAGE 2 Guides:

• Ark Locations Map.
• How to Get the BFG.
• Reduce the Muddiness / Blur of the Graphics.

First Method

• Play Mutant Bash TV. I prefer the Meat grinder, because it’s the most fun. I select “Absolute Explosion”, because its fun and fast.
• Buy all ammo types. It’s advantages if you have unlocked all weapons, so you can buy all ammo types.
• Fast travel to a trading center. Yes, you can fast travel in this game, if you hoover over a location and press F
  (only works if you are outside).
• Sell all your ammo.
• Repeat!
• If you bought all ammo from 1 Mutant Bash TV, buy the ammo from the swamp MBTV.

Second Method

• Steal cars near a trading center.
• Unlock a perk to get double of the cash.
• Repeat!

I recommend wellspring and then go to the gas station, thats by far the fastest way.