Quoted:
PSA has their premium model on sale for $69 plus shipping.
http://palmettostatearmory.com/palmetto-state-armory-5-56-premium-full-auto-bolt-carrier-group-8779.html

AIM surplus has their nitride 9310 BCG for $90 delivered.
http://www.aimsurplus.com/product.aspx?item=XAIMBCGN3BDG&name=AIM+AR%2fM16+.223%2f5.56+Nitride+9310+MPI+Bolt+Carrier+Group&groupid=723

AIM has their standard phosphate coated 9310 BCG for $80 delivered.
http://www.aimsurplus.com/product.aspx?item=XAIMBCGBDG&name=AIM+AR%2fM16+.223%2f5.56+9310+MPI+BDG+Bolt+Carrier+Group&groupid=723

My preference is for the AIM nitride, but any of the three should serve you well.


ETA: I just saw this model for $65 plus shipping at PSA.  It's 9310 instead of C158.  I have no problems using 9310 as it has a slightly higher tensile strength than the mil spec C158 bolt steel.
http://palmettostatearmory.com/psa-5-56-nato-9310-bolt-carrier-group-untested.html

Quoted:
I bought the AIM standard for $69 shipped today--- did not see the nitride  one for $80 shipped or I would have gotten that one. For a spare BCG for $69 shipped,, that ain't bad though.