PM'd you Jester, but my messages have been slow to leave my Outbox lately.
Just noticed that Aim Surplus has RGun stripped lowers on sale for $69.95 (I didn't check shipping costs)http://www.aimsurplus.com/product.aspx?item=F1RGSL
Rguns isn't my favorite company, but the receiver is a bit unusual in that the fire control markings are a combination of words and pictograms, and also has markings for full auto.
I searched but couldn't nail down who makes these receivers. I did find an 2009 thread on AFRCOM where someone claimed that Rguns staff told him that DPMS is NOT the manufacturer but they are made by the same company that makes them for just about everyone else.
That sounds like Aero Precision to me.