To All,
Sorry O.P. But I predict that with a gap that big, your analysis is faulty AND the little old lady is selling below market, both.
Get away from price or cost per unit using data that does not properly reflect economy of scale regarding other ownership costs. Get away from faulty reported rents. Get out of your car, go door knocking, and start speaking personally to renters of similar properties and find out the real market rents, length of tenancies, and vacancy rates currently are. Take the data and use it to estimate what the MARKET RENTs for your comps should be and stop using the probably false reported actual rents. You'll find your GRM will begin to make sense and you can then repair your income approach. Which, by the way, you cannot use an indicated GRM generated by your software from a four-plex against a tri-plex, or a duplex to a tri-plex. A tri-plex is about the toughest of subjects in the 2-4 arena due to this fact when you have no tri-plex comps. Such a GRM is skewed and will give you bad results due to the number of units being off.
And yes, I'd be terrified to send out a report with a gap that big between a contract price, and my opinion, without being able to explain it during the current state of lending. I would not sign and send until I had the explanation. Otherwise, you can plan on getting nailed for not "analyzing" the sales contract versus your opinion of value. Not that I agree with that concept, because I don't.
Webbed.