On average, listings lease in the $20's per square foot per year. The La Mesa region is going through a decline in rates. In the last 3 months, the median rate fell $2 per sqft per year. That's a 9% decline in rent.
Compared to the rest of the San Diego market, the La Mesa submarket has lower rates, so you should have no trouble finding a great deal in this discounted submarket.
In the last 90 days, 11 listings have been listed, which means listings 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 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 2,090 sqft. Most properties in this area are approximately 2 stories. Most buildings in this area were built in 1979 and are Class C.