There is a whole bunch of prep work needed to finish a garage like that. The drywall guys do as little as possible to get by, so you're painters will indeed have a whole lot of work to do. Just hope they know how.

The unpainted garages I have seen have had the basic mudding done between the drywall boards and the nails have been quickly covered over with drywall mud (that's the spackle you are seeing). Based on estimate I have gotten from painters, yes, there is $1000 worth of work to do this at near to the quality level of the inside of your house.

I don't know what level of perfection you want, but you should be able to get someone in to just spray paint two coats of paint (or one coat primer and one coat paint) onto the walls and ceiling for less than $1000. My gut feeling is that it's not going to take 10 gallons to cover it.

You could save a lot of money if you did it yourself with a roller. If you have never painted before, this would be a great project to start with.

I painted my 27 x 27 garage - I'll tell you it's a lot of work - that ceiling is really big - especially when using a roller. I don't think $1,000 is excessive.

It's really nice to have it painted, rather than just bare drywall.

That sounds cheap to me. The garages are just fire taped to meet code, which means they need two more coats of mud with sanding in between coats if you are looking for the type of finish that the interior drywall normally gets. A normal 2 car garage can be sprayed by a painter in about 4 or 5 hours and normally I will use 5 or 6 gallons of paint. Without a sprayer, it's probably a day job for one man. I don't know where they are getting five gallon buckets of paint for $40, but that's some cheap paint there. The bulk of the cost is in finishing the drywall though.

Well thank you everyone for your thoughtful feedback! I'm realizing now that this is indeed a very labor-intensive job.

five gallon buckets of paint for $40, but that's some cheap paint there

wow, missed that, if true, I would without a doubt, not recommend it

$140 would be cheap

Just out of curiosity, I called an outside painting contractor for an estimate. He just emailed me with a price quote of $2,000! (twice the amount the builder's painter is charging me). I couldn't believe it. But still to try lower than $900 estimate, I'm going to ask him what if he just does 1 layer of mudding and sanding, as opposed to 2 layers. I'm not necessarily looking of the garage drywall to look as perfect as the interior of the home. I just don't want the garage to have that unfinished look that most new constructions homes have with the unfinished drywall and the spackle lines all over the place.

For $900 you could buy a compressor and a paint sprayer and a few buckets of paint. Learn how to spray paint.

If all you want is to cover the naked look, spray one coat of drywall primer and one coat of white flat paint. Skip the sanding and smoothing that an interior wall needs. It's a garage!

OR - buy a lot of pegboard, paint it white and cover the walls with that.