I think the payouts are not bad, once you get into the higher level events they pay pretty well, 50k+ for 5 laps on Motegi Speedway, 133k for 10 laps on Ascari lots of 4-5 lap races that pay 40-50k as well as championships with bonuses for winning the lot.
I ran one of the championships in IA yesterday and it was 5 races 16 cars in them 5 laps on most tracks and 2 laps on the ring each race paid near 50K and then a 70k prize for the championship means about 320k for about 22 laps spread out over 5 tracks. Not bad at all.
So far I have managed to get 38 cars + the 25 AE cars for a total of 63 in the garage, have upgraded and tuned a couple of them and still have a bit over 600k left over, the stats show me as having earned almost 3 mil over the last couple of days and I have spent a lot of that time running the license tests and all of the first 2 tiers, after that I just did enough in each of the other tiers to unlock the license test and check out the higher level races.