As stated in at least one forum where this has been posted, it is a Brazilian Baker Rifle, not a Brown Bess musket. The lock mark denotes Pedro I who ruled the Empire of Brazil from 1822 to 1831.
Brazil separated from the United Kingdom of Portugal, Brazil and the Algarves (1815 to 1822) in 1822 but had been a Portuguese colony from 1500 to 1815 and Portugal is/was Britain's oldest ally. They were supplied with weapons by British makers for their military and obviously Brazil continued the practice of buying arms from British suppliers.
This was never a British rifle, it was made expressly for Brazil to the British pattern. As such, it stands separate from British guns when assessing value, Portuguese and Brazilian guns of this period are scarce to rare or at least not often seen but all things being equal (condition, etc.), a British gun will usually fetch a higher price since their are more collectors of British arms. What that value is will be hard to determine, it is certainly beyond my means.
