Obviously it depends on what you build.
I built a 22x22 Craftsman style stick framed 2 car garage with heated floor for $15K doing all but the foundation myself. This was 2 years ago and plywood is quite a bit more expensive now.
Last year, I helped a friend build a trussed 23x28 with 12 foot walls (for a lift). He spent more than double but did quite a bit of yard work, landscaping and a driveway, several retaining walls, etc.
So it's really all over the map. Tabs' range is pretty accurate. It depends a lot on materials and who is doing the work, you or a professional.
As far as some of the comments about code and permits....it's not as difficult as most would make it seem. A trussed garage is a STFI permit - Subject to Field Inspection. It takes about 2 days to get this permit and you'll have three inspection...okay to pour, framing, and final. A garage doesn't need electrical (although I'm sure you'll put it in) so you can get the electrical permit separately at a later date.
There may be total lot coverage issues if you have a small lot and a large home. The city has helpful (yes...suprise) client assistance memos on the issues you'll need to address. Specifically CAM#220.
Look here:
http://www.ci.seattle.wa.us/dclu/camlist/camlist.asp
Yes your taxes will go up. Yes you'll probably need a new footing for the walls....but Legin is correct...no reason you can't use the slab and pour footing walls/stem walls on the perimeter.