Rich, I agree.
This appears to be a really nice 1973, 74, 75 or early 76 Case 446. All the tin appears to be straight and rust free. Everything seems to be there. The only incorrect item is the seat, from a restoration POV. This would be a very easy restoration as a result. Take it apart, refinish all the parts with new paint and put it back together again, replacing items that are worn out.
Once again, the value issue relates to the claim of a rebuilt engine. That's something that needs to be tested. But let's assume that the blade is in nice shape. It has to be worth $200.00 and the tire chains are worth around $50.00 if they are in good shape. So, I don't think that $650.00 for a nice example of a 446 with a rebuilt Onan is too much to pay.