I wouldn't give you $50 for one. They might be worthwhile as collectors items, but you probably wouldn't be happy with one as a shooter, and in my opinion, they are not a good base to build upon.
Unlike the 1911s, the M-9 is not a gun that ages well or can be rebuilt nearly indefinitely. The gun was designed to go about 15K rounds. Then the locking block will usually break. While there are still at least some (MEUSOC) 1911 slides and frames in use from the USGI purchase years (which ended in 1945/46), you cannot continually rebuild on a Beretta At some relatively low number of rounds (usually inside of 30-40K), the slide and/or frame will generally crack.
Yes, 30-40K rounds is a lot, but not for guns that have been in service for twenty odd years, or involved in multiple prep cycles for combat deployments.
There are competitors out there with high round counts on Berettas, but remember, competitors don't generally shoot military ammo when they shoot factory ammo at all.
Here's Ernest Langdons maintenance program. He says 20K with no parts breakage, I think that's at the outside, and not with a gun that has shot exclusively GI ammo.
http://www.findarticles.com/p/articles/mi_m0BTT/is_167_28/ai_110457285#continue
Although his dollar figures are not extravagant, the GI M9s generally got NO parts replacement, unless they got a new locking block pounded in with a rubber mallet when the old one broke, as opposed to the fitting that should be involved. That's NO, as in NO spring changes, NO detail strip cleanings, NO new parts. Also, "rebuilding" the top end is a fairly involved process. It has on at least one occasion involved me calling Beretta and saying "Yes, ma'am, I need to order the following parts."
Incidentally, Mr. Langdon now competes with a SIG.
The 92 series is a highly reliable, highly accurate double action pistol, so long as you can keep it properly cleaned and lubricated and have good magazines. Most owners will never have a problem with their guns.
However, the open top design lets too much crap in for real field use, and the durability issues make it unsuitable for high round count applications.
In the military guns, the combination of field use, lack of higher level maintenance, high round counts, and hot ammo, means the guns in the arms rooms right now are pretty much used up.