CMMG offers the $599 Bargain Bin, and there are stocking retailers in Missouri. Other's put guns on sale from time to time, too.
M16 kits are out with new barrels and a semi auto lower, less than $500. Used, yes, milspec, yes. They are actually government accepted parts, which are the ONLY true milspec parts on the market. As a collector item and shooter, every bit as good as we can get if the CMP could sell them - and they apparently never will.
Ar15.com is a good source for checking bargains, try the you build it forum for the cheap parts thread, and the early M16 forums for sales links. It's like looking for a car, everyone advertises, but you have to get the classifieds and go look - daily - or you miss the deals.
Palmetto has lowers for $49.99 today for Fathers Day. Offers like that last hours, not weeks.