Most listings are in the $20's per square foot per year. The Mandan submarket 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 6% increase in rent.
In the last 90 days, 2 listings have been listed, which means new listings might be going fast. You'll want to be prepared to make an offer immediately. About 50% of the listings in the Mandan 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 1,200 sqft. Most buildings in this market were built in 2003 and are Class A.