Most listings are in the $20's per square foot per year. The El Cajon submarket is experiencing a a rise in rates. In the last 3 months, the median rate went up $10 per sqft per year. That's a 51% increase in rent.
Compared to the rest of the San Diego market, the El Cajon region has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 21 listings have come on the market, which means leases most likley are going fast. You'll want to be prepared to make an offer immediately. 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 1,300 sqft. Most properties in this region are approximately 2 floors. Most buildings in this market were built in 1978 and are Class C.