Most listings are in the $10's per square foot per year. The El Cajon region is experiencing a decline in rates. In the last 3 months, the median rate fell $4 per sqft per year. That's a 19% decrease in rent.
Compared to the rest of the San Diego market, the El Cajon region has lower rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 16 listings have been made available, which means 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 710 sqft. Most properties in this area are around 2 stories. Most buildings in this area were built in 1977 and are Class B.