I wouldn't call them rare. Likely a lot of rocklock hunters have found them to be the great choice they are for deer hunting, and probably seldom cut loose with them. No idea now, used to be states you had to use flinters for deer hunting. IOW, true primitive hunting, not the joke it is now days (same as archery).
Price is relative to where in the country all else being equal. $300 would be very cheap here (literally give away), spot on many other places though, possibly even high at others where they grow on trees.
My switch barrel/switch lock Hawken has a 45 flint barrel to go along with the 36 & 50 cappers. The 36/50 are both set up for long range... if I still hunted deer I'd use the 45 barrel (or my CVA Mountain Rifle 45).
HTH