Most listings are in the $10's per square foot per year. The Imperial Beach submarket is going through a an increase in rates. In the last 3 months, the median rate rose $2 per sqft per year. That's a 9% rise 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, 3 listings have been made available, which means spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,100 sqft. Most buildings in this market were built in 1975 and are Class C.