Trying to determine what is a reasonable amount of interest to pay the seller

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I've been researching what the "going rate/average rate" of interest is being paid in MN and WI when asking the seller to do the financing.  In MN, for a Contract for Deed, all I can find is that it is negotable, whatever buyer and seller agree upon.  Are there any seasoned investors who can tell me what is reasonable?  Also is it more or less if you are asking for a 2 year verses a 5 year balloon?  Your advice is greatly appreciatedl