Commonly, if a commercial lease is advertised as 1500 sq ft at $12 a sq foot, is that $18,000 per month or year?

Commercial leases can range in cost significantly depending on the location as well as the size. 

 If you're looking at a 1500sq ft area at $12/foot then it should cost you 18,000 a year. Your monthly cost will be 1,500. They use the full year cost because you have to sign a lease for at least a year. That cost could also include some utilities or security measures such as electricity or a security desk or guard.