On average, listings lease in the $10's per square foot per year. The South Portland region is going through a a boost in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 7% rise in rent.
In the last 90 days, 20 listings have been listed, which means listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 5% of the available space in the South Portland are subleases, so you will probably need to sign a longer term lease with the landlord directly. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
Even though the median size is 2,730 sqft, there are still 0 spaces below 2,000 sqft. Most properties in this area are around 2 floors. Most buildings in this area were built in 1974 and are Class A.