Freak,
The original J.Browning rifles come up from time to time at auction and it depends a lot on condition and features. Browning Sr. was also an inventor and his multi-chamber guns are the most sought after and can bring 5 figures at auction.
His most well known guns are the Mountain Rifles, and the Browning Arms Company marketed a line of reproductions some years ago.
What's it worth? It was delivered to my Grandmother's Grandfather in the winter of 1844-5 and has been well documented during the 170 years it's been in my family. I am the 4th owner. I suppose if I make it to the nursing home I'll be receptive to trading it for a 5th of Jack, some viagra and a couple of hookers. Get back to me in about 30 years.
As with many smiths of the time, the rifles were hand made and no two were exactly alike. Many, such as this one, were custom ordered with features unique to the customers' needs and tastes.
This rifle is .54 caliber, a very popular round for taking the larger game encountered on the plains and in the mountains of America's 1840's frontier and was produced by Browning in his shop in Nauvoo, IL (which is now a museum). As he completed each component he placed his mark. He produced all the components except the lock which was acquired from another smith who specialized in making and selling only high quality back locks -- a common practice during the period. A back lock was high technology of the period, and fairly expensive and complex to produce.
As you can see, the rifle has not been cleaned or restored. His signature is visible, but mostly obscured by rust, forward of the nipple on the top flat of the barrel.