Alternator Problem

[DesertEagle]

1991 318is

My alternator does not charge the battery, and the battery light in the instrument panel does not light unless I disconnect the wire from the D+ terminal and ground it to the alternator housing.  I've tried a new voltage regulator, and it does not help.  Since only one of the housing screws will budge, I've looked into a replacement.

According to realoem.com, there is an 80amp and a 90amp alternator for my car.  See http://www.realoem.com/bmw/partgrp.do?model=AF93&mospid=47305&hg=12&fg=22  
What I don't understand is at the 3 sites I've found that sell both, the 90 amp is always cheaper than the 80 amp (Bosch remanufactured alternators).  I try to be an educated consumer, and I'm hesitant to make the purchase when I can not explain something that seems so illogical.  What am I missing?

Here's the prices I've found:
Autopartswarehouse.com     80amp $250 (+ $72 core credit)
                                       90amp $157 (+ $100 core credit)

Alleuropeanparts.com          80amp $216 (+ $92 core credit)
                                      90amp $188 (+ $100 core credit)

Rockauto.com                    80amp $247 (+ $92 core credit)
                                       90amp $215 (+ $100 core credit)

[locknload]

Possibly the 80 amp was limited to fewer models or the 90 amp fits lots of other cars. Either case would affect the price of replacement components.

Helpful tip:  if you can afford to go without the car for a few days, it's often cheaper to take your bad unit to a local guy and have him replace the brushes/bearings/voltage regulator.  Drive it in first & he can order the parts in advance.