One has no idea if a bolt is good or bad based on an MPI test. I have found many bolts out of spec dimensionally that have passed MPI but I am not aware of any in spec bolts which have failed MPI. You would be much better off taking that $20 and having someone spend 30 minutes gauging as many critical dimensions as possible or doing an extended firing/function check. MPI is something Colt got stuck with contractually. The HK416 and Sig 551 - two awesome rifles - are not MPI as far as I know. If you want an M4, buy a Colt M4. Nothing else is an M4. If you think MPI is important for non-M4s, then better have all the pistons in your car MPI and send your 416 out to a lab. Or you could just shoot it.
Let me explain this another way… Everyone has a QC procedure (or lack thereof) and QC budget. I am not sure exactly, but say we spend $50 per upper for QC. If we consume $15 of that on MPI, then that means less of the budget can be used for test firing and gauging. It is easy to just pay the $15 and get to the left of the chart. The chart has no column for number of rounds used in test firing, how many dimensions are gauged, how much high speed video is done, or how the parts are heat treated.