Bellows life is a function of time as well as hours. Also keeping the drive up in the trailer position when not being used can kink them as well. 3-5 years is a good range.
B-III bellows run about $ 200 - $ 300 for parts (low end DIY, high end boatyard markup) and 3-4 hours labor. However, the drive has to come off and this becomes a good time to inspect and/or replace other stuff like the water pick up hose and check u-joint, and gimbal bearing etc. The water pump for a BIII is on the engine, so no need to service with a bellows job, unlike the Alpha drive that has the pump built into the drive. Trim sensors can also be replaced with the drive in place. Influencing things is also whether it is a trailer, or dry stack boat, or whether it is a wet slip for the season. With a wet slip boat there is a tendency to aggressively PM while the drive is off to avoid mid-season haul out expense, so a bellows job can get a lot of add-ons.
Henry