If you're just looking for a basic M16 (20" barrel, rifle length gas block, etc), it's hard to beat Del-Ton on price.
1) http://www.del-ton.com/Rifle_Kit_p/rkt102.htm - $465
2) http://www.del-ton.com/Rifle_Kit_p/rkt106.htm - $465
They sell a few other kits, 20" and 16" barrels in any basic AR contour. All you need to complete it is a stripped lower.. you can do what I did and for name-sake buy a Del-Ton lower for $120
Or if you want one of the more inexpensive options you can use:
http://www.karrisguns.com/new-ar15-s...lower-receiver - $59.99
http://www.karrisguns.com/ar-15-lowe...selector-marks - $64.99
http://www.dsarms.com/DSA-ZM4-Stripp...ctinfo/DSZM4R/ - $75
http://palmettostatearmory.com/1560.php - $79.99
After shipping and FFL fees..you're between $550-575. The DelTon kits are top notch. I don't know how long their wait is now, but I remember when I bought mine I had to wait a month, and that was at the beginning of last year when they were still taking heavy orders from the Obama election scare.
If you read reviews, the only complaint you'll probably get is people trying to shoot cheap lacquered steel (Wolf, Brown Bear, old surplus, etc) out of them, and yes at first there can be problems. If you also read, most of them were complaining about getting brass cases stuck after shooting the cheap crap. I have a 16" M4 profile kit from them, and I too decided to try Wolf when I first got it. The extractor pulled the rim off the case, and it got stuck in the chamber. Thankfully all I had to do was use a cleaning rod and bang it out. I haven't shot Wolf since, but after about 2-3k brass cases I tried some TulAmmo, and it's ran great through it as long as I remember to clean it after shooting a few magazines.
Accuracy is 1.75-2.5MOA with handloads (just generic bulk loads), and I could consistently hit an 8" plate at 500yds with the stock trigger and configuration.
Or you could go with a used brand name one, but I imagine you'll still be in the $600-750 area.