You looking to buy one pre-built, or assemble it yourself?
I just built a Rock River Arms 9mm. Just waiting on the upper to get here. I figured if it was good enough for the DEA, RRA was good enough for me. I did a lot of reasearching before I picked that. All told I've got exactly $816.56 in in it -- including taxes, shipping, and background check. Every part is authentic RRA.
All stripped forged lower recievers are about the same, so it doesn't matter what you get. Most folks just get the one with whatever roll stamp they like.
If you don't care about matching all the parts, and you are okay with the roll mark, you can save a few bucks by getting a "Stag" brand lower for about $100. A 9mm upper is $430 shipped. Then you'll need a 9mm mag block, $115, and a lower parts kit (anywhere from $35, to $65).
I also opted to go with the 9mm buffer, but it's not completely necessary. It will reduce wear over time though. That's an additional $25. Then you'll need whatever stock you want to put on it (4 pos. collapsible (like the M4s) can be had for about $35. 6 pos. are a little more, at about $60-$70.
This gun would have cost me $1200.00+tax if I'd bought it prebuilt. It took me about 30 min. to assemble the lower, and I had never done it before. The upper just snaps into the lower like any other AR. Drop it in, and push two pins in.
This kit will accept Uzi mags with a little modification, or will accept these mags without modification. I ordered 3 of them. I've heard good things about them.
You can go even cheaper than that, but that's the route I went, and IMHO, it was the best I could have done.