general rule less than 1/2 of new
Then based on condition, accessories like mobile bases, extra blades, lack of rust, go up from there in $50.00 increments. Also depends on area and demand.
A used 6" jointer should ball park around $200 or less. I paid $225 for a really clean Jet 6" 2 years back. It was in a basement shop, not a damp garage and worked flawlessly. The color was blue, not white
if that helps with age.