Jim243,
I'm going to a gun show this weekend and hopefully will be able to spot a M&P15 Sport for $499-$599 (that's the price range I've been getting). If I can't find one or anything around that price range at the gun show, then I'm going to start building one (not all from individual pieces, but I'll probably do a complete upper, and build the lower. As this is my first one, I know it's not going to be superior, but I'm just starting out and don't need a superior rifle right now. Hopefully after I get more comfortable with the AR and learn more about all the parts, how it all goes together, etc., then I'll build my ultimate one.
Thanks everybody, keep the suggestions all coming. They are all very useful.