You can get a stripped flat top upper reciever and just swap that out. That runs in the 80-100 range usually. Yes there will be some light smithing required, but it is almost as easy as putting together Legos.
RIA 1911A1, G19, P22 3.4 & 5"
Remington 870 Wingmaster Police, Remington 700 SP .308, 3 awesome ARs in the build phase