On average, listings lease in the $10's per square foot per year. The South Portland submarket is going through a an increase in rates. In the last 3 months, the median rate went up $1 per sqft per year. That's a 7% rise in rent.
In the last 90 days, 20 listings have been made available, which means new listings most likley are going fast. You'll want to be prepared to make an offer immediately. About 5% of the available leases in the South Portland 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 available, so finding a space for a small business might be tough.
Even though the median size is 2,730 sqft, there are still 0 office spaces below 2,000 sqft. Most properties in this region are around 2 floors. Most buildings in this area were built in 1974 and are Class A.