TexasCHLforum.com

Most of the manufacturers buy their lowers from a limited number of manufacturers (Aero Precision, Mega etc) - they have them made and stamped with their logo. Providing you have an in-spec lower (it is rare to find one that isn't), paying more for a bigger name in most cases means you are paying for a logo and nothing more...

I agree with the above posters regarding PSA (who's lowers are made by Aero Precision last time I checked) - they have a good product, and stand behind it. They also do frequent sales, and many of their uppers use FN barrels that I've been very impressed with. The downside with PSA is they can be slow to ship at times, and I've had one trigger from them that has needed a fair amount of work to smooth out (seems to be the exception, not the rule). You should be able to put together a decent M4 style carbine from them for under $700 quite easily.

I've heard good things about Spikes also, although I'm not that familiar with them.

If you are prepared to buy used, and don't mind owning a franken-gun, you should be able to put together something for even less (I've got an semi auto M16A2 I built from a parts kit a year or so ago, have less than $500 in the gun, everything except the barrel, lower and sling are Colt).