Its not so much a cheat, more an easy way to get the money for that "special" car you know you want...
There's a race east of the capitol city where you are to race against traffic and reach 170mph. It gives you $15,000 per win.
All you need to do is buy a really fast car (a Saleen maybe?) stay in the outermost rightside going against traffic and you wont have to zig-zag to get that price money.
I'd say you can do 4-8 races a minute - you do the math.