With all due respect, unless you have engineered a specialized kit for a relatively small market yourself. Tried to produce, market and sell that kit, and maintained a business with all the necessary overhead, etc. Again, all for a relatively small market.
Unless you have done it, you have no clue what you are talking about...
Why gon't you go out and "just get a manifold made and intercooler piping done".
Then buy the components (intercooler, turbo, oil lines, fasteners, gaskets, hoses, injectors, etc).
Oh, and don't forget about how you are going to TUNE it. And make sure you do it right (which is very unusual).
Do all that, and then get back to me about how much of a rip-off BMW forced induction kits are...