I would venture to guess that most homes are over assessed currently. Market prices dropped faster than assessments. My assessment went down last year for my home. Not enough. I grieved my assessment and had it lowered an additional $110,000. This is going to reduce my taxes by thousands every year. This is something every homeowner should think about.
In your case, if the appraisesd value is less then you agreed to pay then renegotiate. If you haven't signed contracts you can renegotiate or walk away from the deal.
That said, 3000 sq ft and 50 acres sounds like a steal at 415k, and a deal at 550k, although dutchess country is pretty far up there, so maybe not. When your realtor ran comps, (although with that much land, there may not be too many), what price area did they come up with?