Requirements
Need a Co-op friend at first
Need at least 11 item slots, 12 will give you more.

First you need to find your Co-op partner to get the bargaining magazines which offer 10% off items in the pawn shop.

Bargaining Magazine 1 - Americana casino, roof of small north eastern room

Bargaining Magazine 2 - Food Court, above Speedy Expresso. Use the snack machines next to Lombardi's to get it.

Next your Co-op partner needs to duplicate the magazines to you until you have every spot in your inventory filled with them. Next head over to the pawn shop with the SUV key, normally it's $2,000,000 but since you have the magazines you get a % off. 11 Magazines will get you 110% off, so when you buy the key, you will actually get paid $200,000. If you have 12 magazines you will get 120% off, so when you by the key, you will actually get paid $400,000. Just keep hitting B to buy the key and your money will rack up in no time.

I even screwed around a lot and still managed $950,000,000 by the end of 1 playthrough. You also get bonus PP for you're money, so if you make 900 Million, you get 9 Million PP at the end of the game. Hope this helps.