How much is a 12 bore shotgun side-by-side shotgun?

Depends on condition, make, model. I think you mean 12gauge, not 12 bore. Some have a good resale value if it has exposed hammers, the condition of the bore. General value $100.00

They can run 1$00 for old junk to $100,000 for English high grade engraved case works of art.