Some of the building cost calculators are okay for a rough idea, probably better than here because they are specific to a region.
http://www.costtobuild.net/
http://www.homeadvisor.com/cost/garages/build-a-garage/
http://home.costhelper.com/build-garage.html
I input the square footage and just the basic level of finish and here's what the first link came up with.
this costtobuild.net one is cool. didn't know they had something like this, not sure how accurate it is, but seems pretty close, thanks!
