You're not measuring the seat correctly. You need to measure from the middle of the back diagonally to the middle of the nailhead on the side of the pommel. It looks like your saddle would actually measure a 16.5" or a 17" if measured correctly.
The tree appears to be a wide tree or medium-wide tree.
The saddle itself is a "starter" set, made in Asia. Realistically, with the irons and leathers, I'd say it's worth about $50-75 in our current saddle market. I can buy that same saddle new, just the saddle, locally for $89 plus tax.