If you want a 9mm sub gun thats low cost the M-11/9 or the MAC-10/9 is as good as you can get. There very reliable, there small and you can do just about anything with them.
LOTS of parts, conversion kits, slowfire kits, .22 conversion units and updated parts everywhere for them.
I would say an M-11/9 over a MAC-10/9 because the mags are cheaper $19 for zytel and 23 for converted PAM steel mags. The MAC-10/9 DF mags are anywhere from 50-120 each.
Only trouble ive ever had with mine is keeping it fed as they are fast.. 1200-1400 RPM is about avg on them and with a can its even faster.
Currently there avg price is $3,000 up but sometimes you can find them for $2,800.
Uzi's are another good one, you can find them for $7,000 up. There a pleasure to shoot, parts everywhere, slower rate of fire, cheap well made mags and they just work. Get a registered receiver if you ever want to use any of the .22 kits.
Stens are good guns, shoot well but not much in way of what you can do with them other than the rare 7.62X25 kits that are awfull to fire. The mags are CHEAP 3-10 bucks but Super hard to load since there single feed.
Or just go all out and get an M-16. There most fun you can have with your pants on... and sometimes more.. starting at $10,000 But sadly.. worth it.
Dont expect to get away cheap with any full auto now matter how little you pay for it... Once you get them you will want Tons of mags, a can, spare parts, new uppers, optics, stocks, refinishing, extra barrels, shirts, hats shoes, bottle openers, posters, mag pouches and oh yeah.. copious amounts of ammo! You get straaange looks when you go to Walmart and the guy asks how much 9mm do you want? and you say All of it.. And do you have any more in the back?
Buy them while you still can...
Want to know anything else you can ask here or PM me. Or head over to Uzitalks MAC forums.