Most listings are in the $20's per square foot per year. The National City region is going through a a boost in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 3% rise in rent.
Compared to the rest of the San Diego market, the National City region has smaller rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 6 listings have been made available, which means new availabilities 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 posted, so finding a space for a small business might be tough.
Even though the median size is 59,040 sqft, there are still 0 leases below 2,000 sqft. Most buildings in this area are approximately 3 stories. Most buildings in this market were built in 1974 and are Class B.