My garage painting thus far: last year, I rebuilt parts of the garage since it was falling down, replaced a bunch of rotten wood (including the sill plate on 2 sides), etc. At that time, I painted all the new wood + scraped, cleaned, primed, and painted one side.
I'm doing the other 3 sides this year. These sides were peeling, lots of chips, etc, except that some idiot slapped a thick coat of cheap paint on without scraping (the side I did last year didn't have that coat of paint). Basically gluing all the crap in place. So I used chemical stripper and scraped off the paint, section by section. Then I powerwashed it all. Now I'm scraping it again to get off the stuff that powerwashing loosened but didn't blow off. Then I will need to lightly sand the whole garage (normally I wouldn't, but the powerwashing did some damage and sanding will get rid of the fuzzy wood). Then hose it all down. Then I can prime and paint. I've been working on this since May, I should be done by October.
This is why your painting estimate is so high. The painting bit is easy. It's the prep. And if you don't do the prep, then it'll look crappy and not last. Once this is finally done, I should be good for 10+ years.
The house has aluminium siding. I'm not willing to spend the money to put siding on the garage, the whole thing should be torn down and rebuilt anyway.