There is not a blue book that lists tractors, so alternatively a look at previous sales can provide you some guidance. Using fastline.com, where you can buy and sell used tractors, we were able to find 6 previous sales of a Kubota B3350.
High Price = $24,500.00
Low Price = $13,500.00
Average Price = $19,383.17
Additionally, consider contacting a Kubota Dealer to find the value of your tractor since limited information is available online.