The problem with group buys is development, the under drive pulley is cheap because you didn't pay for the r&d for it. If you want a company to just make you headers, there could be over $1000 just in making a prototype to build the copies from, then you also need to write a cad file, a jig, and / or something for the pipes to be bent the same way, etc.

If you want a successful group buy you either need to have the parts of the ability to design /build them.