This depends on how much you want in your addition. My experience is between $85 and $145 a sq ft. Every material you select will come in a variety of grades and that is what ultimately determines the cost. If you have every visited a "new home" model you know that what you see is generally an upgrade or "in addition to" the list price. I always suggest that my buyers get at least two if not three estimates. Most contractors will do this for you. Also if you rough out what you have in mind you might think about taking the plans to Lowes or home depot to see if they can give you the interior costs. For instance, if you are adding on a bedroom and bath you can get the exterior estimate from a contractor but also check out the cost of the interior (fixtures, flooring, etc) from a Lowes or HD. I checked and there are no estmated cost scales for MN available. So you will have to do your own research. Last piece of advice. Make a list that you want the contractors to quote on. Hard costs (materials), labor, time of job, total cost and talk about a penalty if the job runs over more than 10 business days. Naturally you have to take into consideration the weather but if the exterior is competed there is no reason for workers to be anywhere but finishing your job. Good luck.