You are able to get about $170k in a spectator event in League 6. It is the only one there so it should be easy to find - it is about 3/4 of the way down the list of challenges, pick the one with the least distance (3.9 iles or so).
Once yo have entered the event, pick the last guy - it should say in the box "bet:100 win:2000"(something like that) and place a bet of $8,500.
Now the potential profit should be 170k. The rest is simple, give the guy nitrous when he needs it and he will be in first in no time.
It is a relatively short race so this is very helpul for the higher leagues (5, 6, 7) or even if you are broke.