It appears to be a percussion fowler. From the photos I can't tell the guage or country of origin but if british, there should be some crown over X type marks on the left side near the breach. If belgian, look for an ELG in an oval probably on the bottom of the barrel at the breach. From the green tape, I am guessing that the barrel key is missing (which holds the barrel into the stock.
Depends on the inside of the barrel for a price as well as country of origin. My initial wild guess would be in the $100 - $200 range and partly conditional on the lock working.
cheers mooncoon