cost of materials plus an estimate of the labor hours involved. Area dependent on the labor cost. If it take two guys three days to do the work including taping, mudding, texture coating, ask yourself how much your company charges someone else for 3 days of labor. Who is responsible for the painting when done? Include the time to travel, load and unload tools, clean up, go get the materials if you do not have them delivered. That is the lowest reasonable bid you will ever get. Factor in insurance, workers comp, etc., when you figure the hourly rate in your area. This assumes the work area is 100% empty and accessible, power is available etc.,. Also, we are moving into winter - is the garage conditioned, or will it be at ambient temperatures, which means no freezing. Who is paying to condition it while the work is being done so the mud and texture dry correctly? Etc.. If you decide to DIY, can you handle that many full sheets, you will need to rent equipment for the ceiling, and maybe get a helper. How busy are you and how many hours of billable your trade time are you giving up if you do it yourself?