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20 G

To get this achievement traveling to the casino and smashing the slot machines is pretty effective, however I would advise using the machines that have cash in front of them on the floor a few times first as with each quick attempt you will make a profit of $200. You can also smash the two ATM's within the casino for additional cash.

Also, if you have not done so I would advise rescuing the couple in the bar, they reward you with $15,000 for rescuing them, if you leave the safe room and enter again to talk to Gemini in the scrap yard she will give you the opportunity to earn yet another $15,000 for retrieving her gems, this merely takes a quick dash to the roadblock (next to quarantine zone) and you will find them on the floor next to a car. Equip them and hand them to her to get your reward.

This achievement can be done over multiple playthrough(s) (your total cash spent does not get reset) so don't worry if you cant get it done in one.

I believe it also counts cash spent at other venues such as slot machines.