Assuming it is in 80 hour condition, tractor w/mower probably would bring about $10,000-$10,900 depending on location... $9,000 sounds like a nice price for that tractor... Any extras? Canopy, tires, weights, work lights, etc, can change the value... Also look into condition/history of the tractor.
You can compare it to a new model, but the
B7400 lacks power steering and position control, while the B1700HSD has both... both very good features to have. New
B7400 w/54" mower can be purchased for about $11,500-$12,000, I think... remember, the B1700HSD has more features, more along the lines of what the
B2410 offers vs the
B7400. Is mowing your primary use?
B6100E tractor only could be worth anywhere between $2,000 (aged, worn, high hours, etc) and $4,000 (cared for, looks good/great, lowish hours, etc) completely depending on condition and options (hydrostatic?)... attachments on top of that.
HTH...