Some smiths have gotten away with re-attaching the rib in the loosened area. It's a good idea to remove the ribs entirely and clean out the area between the ribs as this area often has rust, which may be the reason the ribs came loose.

Stripping the ribs and re-laying them might cost you $200-300. The barrels will then need to be re-blued. Figure on $200-300 for that.