Well, the guns run from cheap Chinese and Spanish copies for several hundred dollars to fully automatic versions called a Schnellfeuer for about $12,000. If I had the budget, I'd have one of *those*!
I have what is called a "Red Nine", contracted in 1916 by the Imperial German Army as an alternative to the slowly made Lugers. IIRC It's the only version chambered in 9x19mm, most were 7.63mm. The big red nine on the grip was to warn soldiers not to put the wrong ammo in it. The stock/holster attaches to the grip, and makes it a fairly decent shoulder weapon. This is on the BATF's curio & relic exemption list, so it doesn't fall under NFA rules, much like the Canadian Hi-Powers and original Lugers with shoulder stocks. All in all, I think the "Red Nine" is a pretty good set-up. Ammo is easily available and easy to reload (the 7.63 is a necked cartridge, making it more difficult to reload), it's got cool WWI history, and has the added awesomeness of a detachable stock/holster.
My next choice would be a commercial model, just because they are finely finished, as opposed to the wartime versions which could be pretty rough. After that, I'd start looking for a wartime version. I'd try to buy German. I've seen good Mauser shooter grade pistols go for well under a thousand dollars, which isn't that much more than the price you'd have to pay for a trustworthy Chinese or Spanish version.
Whatever you get, be sure to put a fresh set of springs into the gun. From what I understand, firing the gun with wore springs can actually hurt you, as well as the gun. Just replace them. I'd also try to stick with lighter loads if possible. I run fairly light reloads on new springs with mine and occasionally have a problem cycling. I don't shoot it that often and always at the range, so it's no big deal to me.
