Professionally installed polyurea / polyaspartic garage floor coatings are going to run an average of $6 per square foot and up. Epoxy floor coating systems generally average $4 per square foot and up.
If you are good with following directions and have some attention to detail, you can install your own single-part polyurea garage floor coating system for much less $$ as stated by the vendors above. The vendors on this forum are excellent with helping the DIY guy out. In addition, it's actually easier to do than the store bought DIY kits from your local home improvement center.