On AR15.com there is a guy selling stipped lowers for $100. If you order from him and put ARFCOM in the messages, shipping will be Free.

On Mapartsinc.com, they sell a Varmint upper in 18, 20, and 24" with a butt stock, trigger group, all pins needed and bolt for $525.

The last time that I ordered the 24" upper from them, it took a while for them to get it in and ship though. So if you can mind the wait, you have a $625 AR that shoots extremely good! If your friend will transfer the lower to you for free, then that will help with additional costs.