Most listings are in the $10's per square foot per year. The Rincon submarket is going through a a boost in rates. In the last 3 months, the median rate rose $19 per sqft per year.
In the last 90 days, 0 listings have been posted online, which means spaces most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. About Inf% of the listings in the Rincon are subleases, so you might be able to sign a shorter term by taking over a previous lease. 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 930 sqft. Most buildings in this market were built in 2007 and are Class B.