The races don't pay a lot, considering how many things cost money, and how much some of those things are. Yes, there are some events now you can grind for good cash, but even then it would take a very long time if you wanted all of the cars fully tuned. You can't say that the game is about driving the cars you like, not collecting all of them, because the arcade and career are probably the worst in any racing game since gt5, and comes across as a complete afterthought. The most fun thing about the game is collecting cars to a lot of people.
The races that do pay well were also brought into the game long after everyone who had any interest in the game had finished it. Seasonals are all very boring, and only pay well if you remember to log in every day. Which is fine if you play it every day, but some of us have a life, and can't turn the ps3 on every day.
The funny thing is, they had a far better system in gt4, where you could win a large number of cars in the career, which had much more variety and a lot more events. Being able to sell an expensive race car you won, and being able to win cars more than once, was much better than what gt6 has. But Pd being true to form, tried to fix what wasn't broken with previous titles, and left what was broken alone
Essentially they made the credit system in the game very unrewarding to try and push the micro-transactions on everyone. One of many reasons why gt6 has been a huge failure, relative to the other games in the franchise.
Anyway, we're getting a little OT, so I'll leave it at that. 👍