On average, listings lease in the $20's per square foot per year. The Portland submarket is going through a an increase in rates. In the last 3 months, the median rate went up $24 per sqft per year.
In the last 90 days, 44 listings have been listed, so you'll want to check back regularly for new listings. About 111% of the listings in the Portland 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 2,080 sqft. Most buildings in this region are around 3 floors. Most buildings in this market were built in 1900 and are Class B.