The typical asking rate per listing is in the $20's per square foot per year. The Dallas County region is going through a decline in rates. In the last 3 months, the median rate decreased $3 per sqft per year. That's a 11% decline in rent.
In the last 90 days, 2432 listings have been listed, so you'll want to check back regularly for new listings. About 2% of the available leases in the Dallas County are subleases, so you will probably have to focus your search on direct office leases. 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 1,400 sqft. Most properties in this area are approximately 3 floors. Most buildings in this region were built in 1983 and are Class B.