Depends on the make .
A Mc milllan will set you back some serious $
A Reese about $300-400 U.S.
There is a new company selling a modified USGI fiberglass stock with AR folding stock hardware on gunbroker.
Vltor was making one selling for, I believe $450 the last gunshow that I had seen.
I am sure there is a make or 2 that escapes me at the moment . Please feel free to correct me if I left something out .