Building something like that with junkyard parts sounds pretty cool, takes some ingenuity... But I figured your original question was about why some company isn't doing it. I bet the real (new) cost of that APU compressor section was a lot more than $150.

The plastic computer fan for the first stages sounds kinda wimpy to me. Be careful it doesn't come apart in real-life use.

You measured 4 or 6 psi, but I'm guessing that's dead-headed (iow, zero flow rate). What you'll want to do is measure how much pressure it can make WHILE it's delivering over 200 CFM. (2.0 liter, 6000 rpm.) That's exactly why those electric superchargers are such a joke.