Most listings are in the $10's per square foot per year. The Imperial Beach submarket is going through a a boost in rates. In the last 3 months, the median rate increased $18 per sqft per year. That's a Inf% increase in rent.
Compared to the rest of the San Diego market, the Imperial Beach region has lower rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 2 listings have been posted online, which means leases most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 780 sqft. Most buildings in this area were built in 1990 and are Class A.