On average, listings lease in the $10's per square foot per year. The El Cajon region is seeing a an increase in rates. In the last 3 months, the median rate rose $17 per sqft per year.
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 discounted submarket.
In the last 90 days, 16 listings have come on the market, which means office spaces are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 50% of the available leases in the El Cajon are subleases, so you might be able to sign a shorter term by taking over a previous lease. 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 2,280 sqft. Most buildings in this area are around 2 floors. Most buildings in this region were built in 1977 and are Class B.