Play Underground mode until you get the skyline. 
Make sure you have enough money to change to a VW Golf. 
Buy a VW golf then put all your Performance parts on it. 
Once you have done that, trade it to an S2000 and take 
all the parts off it. Once you have done that trade to a 
skyline (it will cost 3000)then trade back to a VW golf 
for a gain of about $11000 (not bad huh?), you can continue 
to do this for as long as you like.