On average, listings lease in the $10's per square foot per year. The Keller region is going through a an increase in rates. In the last 3 months, the median rate increased $16 per sqft per year.
Compared to the rest of the DFW Metroplex market, the Keller region 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 new listings are probably going fast. You'll want to be prepared to make an offer immediately. About 167% of the available space in the Keller 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 1,550 sqft. Most buildings in this area are close to 1 floors. Most buildings in this market were built in 2006 and are Class B.