McDouglas seemed to have a talent for picking oddball engines for the later DC-9 series. The JT8D-209,17,19 is unique to the MD-80, so its costs are much, much higher today than the JT8D-7,9,15,17 that was on the DC-9. The BR-715 on the MD-95/717 was new territory, too.
McD also chose a single supplier for all avionics, APU and electrical for the MD-90/95. The cost of maintenance on those items is at the mercy of Honeywell, nee Allied Signal, nee Bendix. McD built the planes and the single source suppliers have made the profit.
The smartest thing done on the A-320 was to provide a choice of engines. Competition has been so hot that GE & Rolls were giving away the engines on airlines' initial orders. Negotiate to buy a 737 and you're at the mercy of GE.
GB