DIY prices for parts are for DIY. You won't find any kind of real tech who will install those parts. He may have been generous and wanted to see if you had quality parts. Maybe if he would have aproved of the parts he would do such an unbusiness like proposition.
A shop price for repair includes profit on parts that are necessary to extend the warrantee for such repairs. A quality shop will not allow work to be done in its name which can not be fully warranteed.
Bosch Master, ASE Master, L1
33 years MB technician