i have a chance to buy a Winchester 1866 Musket how rare are these?

I know very little about them but looking around online they appear to be pretty rare and are bringing $5,000 plus for one in good shape.

Thanks for your information Homebrew.This musket is in mint shape,I had a gunsmith look it over and he said he had never seen one this nice.It looks like it was bought and then stored with out being used for what ever reason.