The actual materials and installation labor is probably the minimum charge for the contractor to show up plus a few $ in materials - so $50-100 for a handyman or about $100-200 if a carpenter or general contractor.
If easily accessible. If it means cutting through drywall ceiling or wall to install it, could run into quite a few hours given several trips to cut in and install the firestop, wait till it passes inspection, then come back and repair the drywall or flooring that had to be cut through and mud the drywall joints and nail/screw holes, then come back to prime, then come back to finish paint once or twice - so could easilyk get into the $500 or even higher range.
Course, if this was something that the contractor should have done as part of a tub installation say, then should be at no cost to you.
