My Favorite R$ earning race is in the Pro/Am Supercar Club, the very last race is the Suzuka Circuit Cup.   I use the GT-R and get $31,800 (with the 100% daily bonus) so after the servicing I clear $30k each time.    After that I like the pure stock challenge , the last set of races (17)  it is a toss up for me on the Elimination or Hockenheimering.   Both are pretty good payout.  The thing with the Elimination is that you can use the Silva or Focus which split second servicing.