On average, listings lease in the $30's per square foot per year. The Kent submarket is going through a a rise in rates. In the last 3 months, the median rate went up $5 per sqft per year. That's a 18% rise in rent.
Compared to the rest of the Seattle Metropolitan Area market, the Kent region has lower rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 10 listings have been made available, which means office spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 110% of the available space in the Kent are subleases, so you might be able to snag a great spot on a short term. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,330 sqft. Most buildings in this region are around 1 floors. Most buildings in this area were built in 1986 and are Class A.