bikeracer -
Welcome to EM! I have a rule that my mod be at least 95% reversible, which allows for little holes here and there (but not where you'd easily see them). It's not that my car is worth anything, it's just that I want to be able to reverse a mod I don't like.
There are not too many kits out there. There are aero wheel skirt kits for the Prius :
Toyota Prius - Wheel Skirts - Toyota Prius Wheel Skirts by Hybrid Effects 1 pair (Shipping Included)
There are also undertrays for sports cars like the S2000 :
Honda S2000 Diffuser Front Undertray Photo
Undertray stuff also exists for other import tuner cars like the Evo :
Voltex Front Under Tray Mitsubishi EVO VII VIII IX 01-07
The bottom line is not much.
CarloSW2