We don't have a flat number we can post here that holds up house to house - the cost swings on whether you need new wiring run, whether the existing rail and hardware can be reused, and which drive type you pick. What we can tell you is the shape of the decision: a straight motor swap onto existing rail and track is the cheap end of the job, and a full opener-plus-wiring job because the old circuit can't carry a modern unit is the expensive end.
Given the median household income here runs a little under Brandon's broader average, most of our Town 'n' Country customers land on a mid-range belt-drive unit rather than the premium Wi-Fi models with every feature - it's a quieter motor than the original chain-drive, without paying for sensors and cameras most people don't use. We'll give you the real number after we see the garage, not before.