what kind of gun? Is it a converted flint to cap lock?
Any information as to who made it and when?
Rifles are not that rare but they can still coommand upwards of 10K and much more if associated with some real historical prvenance.
Then, of course, condition, condition, condition.
I dont think any of us here are really qualified to givea market stimate (I do some antique "picking" but dont get involved with guns or militariana.
you should post a picture (with something for scale and color)