well I know it's not a stack tube blowback, but it is this months project for me. chem-x sold me an alleycat (in bad shape) at PP3 for $5. so needless to say I had to do something fun with it.
to start with the vert-air adapter was missing. the ICD verts are quite prioritary, so one day at work while checking up on a few guns we had sent out for repair, I asked them if they had any.......they basicaly laughed at me. so i decided to plug the port for the asa:
http://lexiones.googlepages.com/Pict...re018-full.jpg
so after that was done i noticed that the gun was really looking like a pistol, so i just went from there and for $10 I got an ariakon acp feed. i shaved down the springfeed so that it would mount on the the dovetail that ran almost the full length of the gun.
since the gun is powerfeed(and the powerfeed screwed onto the gun) not much milling was required to make it into a vertical feed marker.
http://lexiones.googlepages.com/Pict...re016-full.jpg
from here the springfeed could just slide into place. all that needed to be done now was to tune the blowback and velocity, and the gun was all set.
so if this is enterable i suppose it would be in the under $25
gun $5
stickfeed $10
barrel free(thanks to my boss)
m-16 grip free (already had one)
$15 total
http://lexiones.googlepages.com/Pict...re013-full.jpg 
http://lexiones.googlepages.com/Pict...re011-full.jpg
VVVVV i'm not cutting down a lapco barrel...even if it was free