Really depends on when you start charging, and when it was last serviced. Add haul out, removing drive, impellor only or entire pump, drive shaft oil seal base, splashing boat again etc. What else is done during that time? Pressure check, clean splines, grease ujoints, check bellows, gimble bearing etc. It all takes time. Anything can be done in minimal amount of time. Is it being done right and is proper preventive maintenance being done to stay within a quoted time limit? Especially with sterndrives, I always suggest that in a perfect world, it would cost this much but expect that the unexpected is the norm with drives and that the quote is only a guideline. Ya, changing out the impeller, not much time, it's everything else that could/should be done on the way that will dictate the true cost.