Not sure who's still playing this. I figured out eventually that you get the biggest bang for your buck on mods if you go for the longest race distance, so that's all I've been doing now. Races take usually between 45-60 minutes, which means I can't just sit down and do a quick race, but money is no longer an issue. I finally just bought my first mod crate for 30k. One of the mods out of it is 80% credit bump if you turn off rewind. The first race I did was 30 laps at Laguna, took about 43 minutes (shorter than usual), made 100k off that + 100k for highest difficult + 100k for the VIP bonus + 140k for the mods (the 80% and two 30% ones for things like not using stability control), so a good 440k off that race, plus leveled up a few times, so that's another 60k on top for 500k, and that was a pretty light haul since the race was shorter than most. 80k of that was from that one mod that came from a 30k crate and I've got two more uses on it.
On the downside, the longer the race is, the easier it is to win. I lapped up to 3rd place in that last one and the drivatars are on "Unbeatable". A lot of the single player challenge comes from making you start back in the pack and giving you a limited amount of time to get to the front, so that's gone. On the positive side, I can now afford basically whatever car I feel like getting.