The pistol appears to be similar to a Blunt & Syms Single shot side hammer pistol as shown on page 389 in "Flayderman's Guide To Antique American Firearms", 9th ed.
That doesn't mean it actually is the gun mentioned as the Ethan Allen and others made similar guns.
The value of a Blunt & Syms in 2007 ranged between $300 for a good condition to $850 for one in a fine condition. Based on the picture this gun is not in a "fine" condition.
These prices are for a fully working gun. There are more than a few on the market that do not work like they should so be cautious.