i don't think there is but figured i'd ask if any place has a whole underbody diffuser, and if somebody did how much would you pay for it?

 

As far as I know, there isn't one available. No real point on a street car, and on a race car you'd get mad lift unless everything was tuned correctly.

Though it shouldn't be hard to make one out of a big sheet of lexan or other plastic.

 

if this where to be done, you are looking at thin aluminum (possibly honeycombed), fiberglass, cf, or kevlar. could mix any of those. the design time to clear suspension components, exhaust tubing, and to provide a safe underbody flow, more then likely using the venturi effect to provide downforce is going to be a bit. mounting points of course taken into consideration. this would probably be 3 piece in design as to make it easier to ship, install, and fix. you are looking at well over a grand easy given cheaper of the materials. i would say 2k. this would also have to be designed to fit cars with varying front, rear, and the possibility of sideskirts. not to mention the epoxy/coatings to withstand the normal road debris and small woodland creatures encountered on the roadways.