Ah, I see. Thanks, I figured that could be the only way. Nobody tell Capcom about this, shhh!
The bargain magazine subtracts 10% of the price from everything in the pawn shop. SUV key costs $2,000,000 normally, so with each magazine giving you 10% off it's going like this.
0 magazines: $2,000,000 for key
1 magazine: $1,800,000 for key
2 magazines: $1,600,000 for key
3 magazines: $1,400,000 for key
4 magazines: $1,200,000 for key
5 magazines: $1,000,000 for key
6 magazines: $800,000 for key
7 magazines: $600,000 for key
8 magazines: $400,000 for key
9 magazines: $200,000 for key
10 magazines: $0 for key
11 magazines: -$200,000 for key
12 magazines: -$400,000 for key
For all the positive values you're still paying the shop, but when you pay negative money for the key, you're actually gaining money. In math when you subtract a negative number from something, you're actually adding it. So for example, 12 minus -4 is 16, and your current money minus -$200,000 is whatever your money was plus $200,000.
Nice find TC!