I don't think it is buying an advantage at all. I think you are simply compressing time when you spend money.
When you spend $$$, you are moving days/week/months further along. You are just buying stuff that would have taken longer to earn FTP. It would be an advantage if I spent $500 and played the same person tomorrow with my stronger roster. But I wont be playing the same person, I'll be playing someone slightly more advanced.
Spending the money has a net effect of 0 as far as advantage in battle goes, it just changes the tools with which you fight..