Cheapest build you can do for easy that I have found:
9mm Rock River Arms A4 upper from ADCO (go with postban, as prebans have been on backorder for 2 months or so...) $367 plus $4 shipping (Just under $400 for A2 style)
9mm hammer (or DPMS style .223 hammer unless you already have one) $15 give or take
VM Hytech 9mm adapter block $130 plus shipping from Ameetec Arms on the EE
Uzi mags at $15 each for decent ones (gotta have 4-5!) Try Cole Distributing
Total: $680-700
You can also go Olyarms style:
9mm Olyarms upper $585 plus shipping
SOCOM mag adapter block: $65 plus shipping
Sten mags (4-5, probably a few more to gaurantee "quality" ones) $6 ea from CDNN Investments
Total: $680-700
Some people argue you can do better, but I would say these are "reasonable bargains" that you can always find for prices.
Note that if you just want a pistol caliber longarm you can go with a Beretta Storm for $600 plus spare mags or even a Ruger PC9 for $450 pus mags whereas with an AR you still need a lower to work from.
Not a cheap proposition, but the ability to train with your AR at a pistol range and the cool factor are PRICELESS