On average, listings lease in the $20's per square foot per year. The El Cajon region is going through a a boost in rates. In the last 3 months, the median rate increased $10 per sqft per year. That's a 49% rise in rent.
Compared to the rest of the San Diego market, the El Cajon submarket has smaller rates, so you should have no trouble finding a great deal in this discounted submarket.
In the last 90 days, 17 listings have been posted online, which means office spaces are probably 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 on the market, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,000 sqft. Most buildings in this market are close to 2 floors. Most buildings in this area were built in 1978 and are Class C.