The typical asking rate per listing is in the $10's per square foot per year. The Keller 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 7% decrease in rent.
Compared to the rest of the DFW Metroplex market, the Keller submarket has lower rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 3 listings have been listed, which means spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. 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 2,070 sqft. Most buildings in this area are around 1 stories. Most buildings in this area were built in 2006 and are Class B.