I couldn't venture a guess as to the value of the piece, but I'm sure others can. That said, the lock plate indeed reads "Fairbanks" and you mention "AB Boston" in your title. If you mean to reference A.B. Fairbanks of Boston, he died in 1841 so that should establish a "not after" date; being a percussion gun, it most likely wasn't made before 1825, and probably later than that. If it is a Boston gun, hopefully Joe Puleo can chime in on it.
Jennison