You should be able to find one in very nice condition for about $6k. Prices are held down by the very expensive and hard to find ammo; figure about a buck a round.
At the Hiram Maxim shoot in Maine two years ago, I saw a guy actually FIRING an MP-44; he said the hardest part was finding ammo for it, because the last people to make it were the Czechs, obo. 1955; they were actually issuing these to their border guards for a while.
My Father used to spend hours making specialty ammunition in the Garage. I remember that when it first came out, the 44 AutoMag was expensive to shoot. Heck, it's only a cut-down and then trimmed .308. All you have to do is cut down and trim any standard based round and then run it through a few sizing dies to form the cases. I'd think that brass for the MP-44 would be fairly easy and cheap to make.