Coming from the 335 scene I can attest to the fact that parts CAN be much cheaper.
You can get very decent CF Hamann and BMW performance lips for 250. CF CSL trunks that fit great for >700. CF diffusers for 200.
Also, you can get an M3 Front bumper made of PU with OEM fitment (I've verified personaly) with ducts and screens for less then 500 shipped.
A lot of posters here are mentioning things which already exist. There are plenty of aftermarket diffusers / reps. There are at least 2 icarbon Rep lips out there now. There are CF and FG CSL trunks on ebay now.
Why produce a product to compete with them. The things that I think would sell very quickly and don't exist now (at a decent price):
A replica 3D lip in either PU or CF
A replica 1M front bumber in PU