Caporegime
The rival thing is good though, done a few in the monthly one (the nascar one, can't remember the track) and earn some silly money as I think if I sent a time 0.001 faster than previosuly I get 150,000 credits. Though easier said than done as it took me about 15 laps to get a clean lap together to beat the previous one, still better payout than playing the career normally (or at least the early career that I am racing in)
I think the payout is determined by the rank of the person you're targetting, like for a top 100% time you'll get 500, top 50% you'll get 2500, top 20% you'll get 10000 or whatever... That seems to be how it works.