I found a much quicker a-spec cash grind last night...
In expert series, turbo races (middle one in the grid choice), do the high speed ring race with a maxed out Zonda R. total race time is about 3 minutes and you get $50k.
it gets pretty boring because you have to race the same thing yourself over and over, but if you are just trying to grab some cash really quick it's great. I was b-spec grinding all night till I ran that race and got the last few hundred thousand I needed to get my McLaren F1 in about half an hour.