Why not make one for a Corolla or EG/EK Civic or Metro? Personally I don't think people will shell out ANY money for something that makes their car (and especially truck) stick out. How about offering a kit with:
-variable grille block
-air dam
-aluminum belly pan
-functional diffuser
-clear smooth wheel covers
-vacuum gauge
-removable trunk/hatch mounted kammback
Of course everything would need to be stupid simple to install, or come with instructions for a shop to do it. I don't see a lot of pickup owners going for the full boat tail because not only can't you use the pickup then, it's not "manly" either, defeating the only reason to daily drive a pickup.
Also, if someone's doing this for economic reasons it's going to have to be really, really cheap to save them money in the long run. Maybe offer a driving clinic with the purchase of said kit. This will not only get them better gas mileage either way, but make them think it's the aero mods doing it all!