There are OPE blue books. In my experience, we used to figure that the value of standard lawn tractors would depreciate ~ 20% per year. Accessories carry very little resale value on their own as the beauty is in the eyes of the purchaser, so to speak. This tractor compares to a Cub Cadet that you would find at Lowes. Know that new used values both are lower now than in prime season- so bargains can be had.
Do you homework too. There have been maintenance needs along the way. Have they been kept up on? Belts and blades, the soundness of the drive, possible engine issues etc. This may have a Kohler Courage single which brings the potential for a cracked engine block into the mix.

Since this model has been around a while you might consider searching online for user opinions and experiences.