A similar (or the same?) MAC-11 M11A1 modification to accept Glock magazines which I blogged about back in March is up for sale on Gunbroker:
Link to auction – http://www.gunbroker.com/Auction/ViewItem.aspx?Item=175724611
The minimum bid is $5479, which seems steep to me. I have no idea how much those things are worth, but I think id rather have a few other guns than that one. It has been re-listed on Gunbroker a lot of times.
Hat tip: theoperator