I got a couple million from the seasonal events. They can be annoying as you spend laps trying to shave off fractions of a second. But every lap you figure out where you can save time and eventually put a good lap together. I haven't got gold in any yet but silver still pays well.
I've won half of the Super events and they pay really well (especially with the 200% login bonus). I still have a few events to win in both International categories but I'm doing the Super ones because of the $$$, plus they are challenging and fun. You have to worry about Tyre wear, daylight, fuel usage and the weather all whilst having the AI drivers ram you off the road instead of using their brakes. It's all about being on the right tyres at the right time.
I've spent most of my time racing instead of online:
Novice - 24/24
National B - 90/90
National A - 132/132
International B - 96/141
International A - 40/159
Super - 15/27
Also, I've won both Red Bull Kart + Junior Championship and got gold in the first six events at Goodwood.
Actually, I just did one of the new seasonal events that they added today and got gold by over 2 seconds on my first attempt and made 1.5 million. Easy.