There are different versions of this bayonet & scabbard (models 56 and 58 for the 49/56, and whatever the FAMAS version is called, and maybe more versions -- some of our French members or Buddy Hinton will no doubt know for sure), although I'm not sure how many of us pay attention to that. AFAIK, all versions will fit all the rifles, including the FAMAS. I believe that scabbards are specific to each version of the bayonet.
In the last couple of years I've seen prices range between $25 and $100, at the same time, with no real connection to condition or anything else. I leaned towards the $25 deals when I bought my examples -- all it took was patience.
Right now there are several on eBay for $40 - $100, plus one stripped of the handle & locking mechanism. They all appear to be the later type (model 58). Gunbroker has one, but at $37 it hasn't yet hit the reserve, so who knows where it will go.