Another option that I am thinking about is the MechTech, if you already have a glock (or 1911). Basically remove your slide and install the MechTech "upper" to covert your pistol into a carbine. Looks like its around $500 for a system with stock. Use your existing glock frame (although it requires a filed down mag release to act as a bullet button).
I understand that you can just buy a frame from places like LoneWolf, etc, if you want to make it a stand alone carbine.