You can save money by piecing and AR15 together (lower/upper), but its getting difficult. Putting an upper an lower together takes all of 10 seconds.

I know Midwayusa.com you can buy a few different makes of lowers, and you can buy uppers there.
Ameetec.com
adcofirearms.com (I've used Steve here 3 times for ARs and his service is excellent)

Hope this helps ya out some. You might also want to check the classifieds at ar15.com

Well... I found a lower that I liked. I really like the look of the collapsable stocks. And it's DPMS.

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=723357&t=11082005

As for the upper, I'd like this AR to be versatile likeI said before. I'd like it to be able to use it for varmints or predators. Is the 16 inch barrel really pushing it for accuracy? Would a 18 inch or 20 inch be a lot better decision?

http://www.pkfirearms.com/store/get_items.aspx?type=0&term=77

^If you scroll to the second and third from the bottom, there are 2 20 inch barrels. There are also a couple of 16" barrels near the top. All of which are in the mid $300 range, which I like.

From my limited experience with AR's - there are two distinct reasons not to go with a 16 in barrel - accuracy (they are still accurate tho) and noise - the 16" barrel willbe a lot louder than say a 20" barrel - and if youre worried about overall length - youve still got a collapsible stock that youre looking at - cha shorten it up a lil for whatver reason you want