Colts only put them on complete rifles, the R6700, and only those with large holes. I've stripped them out and sold them for as much as $450.
Armalite used to sell them stripped. Last one I saw the guy wanted $250.
Evidently, Les Baer just put some on the market. I think they were about $170. I don't know about their current availability, but I'd think that would be a good price.