The typical asking rate per listing is in the $20's per square foot per year. The Antioch region is going through a decline in rates. In the last 3 months, the median rate fell $1 per sqft per year. That's a 2% decline in rent.
Compared to the rest of the East Bay market, the Antioch submarket has less expensive rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 6 listings have been posted online, 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.
The standard measurement for office space in this area is around 750 sqft. Most buildings in this region are around 1 floors. Most buildings in this market were built in 1989 and are Class C.