The big commercial sellers seem to be asking $1500-2000 for any good 2-band, 3-band or cavalry carbine but they are always 25-30% high relative to private sales. One of the last ones I bought (less than two years ago) was a super nice 2-band Mk II** (which are surprisingly hard to find) for which a very experienced collector here on CGN only wanted $1100. For slightly rough but sound shooters I suspect they can still be found for $800-1200, as Wildey says above.
If looking for a shooter avoid badly pitted bores and beat up breech-blocks. There are lots out there with surprisingly good bores. Lots of guys shoot the iron barreled , non locking breech Mk I* to II** but a Mk III, with locking bolt, is never a bad way to go. The steel barrels were intended to wear more slowly and were not brought forward due to any weakness issues with the wrought iron barrels. Wrought iron is great stuff- has a certain ductility due to stringers of silicate slag incorporated within.
milsurpo