M11/9 is the cheapest. M10 probably the best way to go if you are going to get a Mac, you can do .45/9mm/and soon to be .22. If you keep an eye on a few different boards you can pick up a M11/9 for $2800-$3000 with accesorries. A Powder Springs M10 can be had for $3000 or sometimes less if you are willing to wait a bit.
If you want to spend a little more you can get a Stemple 76 or a Reising. I would probably avoid the Reising from what I hear they are hard to get parts for and don't run 100% without some tinkering. I wouldn't pay any more than $3500 for a Stemple, I've seen a couple advertised for $3200 here lately. If you have to have a Reising I wouldn't pay any more than $4k. The Spitfire SMG is also in the same price range, these are another I'd stay away from. No parts and lots of tinkering.
As others said, try making some offers you may be able to get a really good deal.