Most listings are in the $20's per square foot per year. The San Marcos region is experiencing a a rise in rates. In the last 3 months, the median rate went up $1 per sqft per year. That's a 3% increase in rent.
Compared to the rest of the San Diego market, the San Marcos submarket has smaller rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 25 listings have come on the market, which means new availabilities most likley are 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 1,160 sqft. Most buildings in this area are close to 2 floors. Most buildings in this area were built in 1984 and are Class B.