I have a '93 Bonneville with the supercharged 3.8 liter engine. When the supercharger started making suspicious noises, my GM service center told me the only option was to buy a new one from them for over $2400 + labor to put it in. In doing some checking, I determined that there is an alternative that is much more reasonable. Magnuson Products in Ventura, CA (tel 805-642-8833) sells rebuilt superchargers for $1,100 plus a $200 core fee (refundable if yours is usable) plus $50 shipping (covers both to you and return of your old unit to them).
I lucked out even better. They had a "cosmetically imperfect" unit for $550 plus core fee and shipping. According to the tech I spoke with at Magnuson, these units are mechanically perfect and he'd use one in his car, so that's what I bought. The "blemish" was that the covers were from different cars and were therefore different colors.
Put it in four days ago and it's working perfectly. I realize that's a short time, so if there's any change, be sure that I'll post it here.
Magnuson also sells "nose cone kits" if only the bearing is bad.
What's really annoying about my GM service center not knowing about this is GM put out a Service Bulletin in 1999 pointing out that Magnuson was a source of more economical units when the customer had to pay for it (as opposed to warranty, where they have to put in a new unit). If anyone wants it, Magnuson can fax you a copy. It's labelled Bulletin #99-06-01-012 dated July 19, 1999. Magnuson is a factory authorized rebuilder (the only one according to their literature) of Eaton superchargers -- which is what GM put in their cars.
I also found Magnuson's people to be really helpful. Highly recommended!
Hoping this helps others in a similar situation.
Martin