I just finished a 750sf garage here in Southern California and the cost was as follows:
Concrete $11,000 (includes required stem wall without would have been $7500)
Plumbing & Gas $1,000
Framing $4,500 (12 foot ceilings)
Lumber $5,000
Electrical $1,000
Door & Windows $500
Epoxy Flooring $1,600...