On average, listings lease in the $40's per square foot per year. The Mercer Island region is going through a a boost in rates. In the last 3 months, the median rate rose $5 per sqft per year. That's a 13% rise in rent.
Compared to the rest of the Seattle Metropolitan Area market, the Mercer Island submarket has more expensive rates, and you'll have to negotiate above asking rent to be in this hot submarket.
In the last 90 days, 1 listings have been made available, which means new availabilities most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. 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 1,460 sqft. Most buildings in this area are approximately 2 floors. Most buildings in this region were built in 1979 and are Class A.