What's this worth...
 

A friend wants to take his C2S back to stock because his lease is almost done. He's remove his aerokit and is interested in selling it to me. He paid $10K new for it installed. I will obviously have to paint it but was wondering what I should pay for the kit.

I say 4k max.. and thats being generous. Thats what I would pay..
Misha has the a replica wing for 1400 and you could probably find a new gt3 bumper for 1700-2000

A high end used car salesperson who had 15 years in the business and I had a discussion once. Upon a trade in he expressed that mods will hold 25 percent of their value on his lot. Using his numbers in trade the part would get 2500 for the 10K .

If the guy is a "friend" my feeling is anything you pay above 3500(retail) is more about your friendship than what the parts are worth. In fact my feeling is generally to avoid business with friensdships --even a simple car part sale .