The typical asking rate per listing is in the $20's per square foot per year. The San Marcos submarket is going through a an increase in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 3% increase in rent.
Compared to the rest of the San Diego market, the San Marcos region has lower rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 19 listings have come on the market, which means leases are probably going fast. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,250 sqft. Most buildings in this market are close to 2 floors. Most buildings in this market were built in 1984 and are Class B.