Like everything else its all dependant upon the condition of it.
What motor is on it, and how many hp?
I am using Cdn dollars so bear with me, with its age, and if it is a basic mill with no add on's or attachments $1-1.5K, in realistic money in near new condition, as new updated models can be had for $4k.
Four to five thousand board feet is nothing its, more about the care and storage the mill had and the owners work habits.
The value of the dollar has changed a lot in the past 10+ years, that is also a consideration.