The current consumer for these types of product is cheap.
So as a business model, you aren't starting out with a potential customer w/ money.
Next, eventhough we aba test this stuff, there is 'some' placibo effect. So uncle bob with his heavy foot isnt going to see much improvement.
now when gas gets to $6, there maybe an increase in the size and type of person in the market for these items.
Infiniti has a pretty nice bellypan on their cars from the factory already. problem is, nobody knows why it's there and they take it off the first time it gets dinged up.
And finally, setting up molds isn't cheap.......
on the other hand........
china has no problem cranking out illegal hid plug n play kits all day long......go figure