Sorry we can't advise you on that as there are too many factors to consider, such as the age and condition of the saddle, the current demand, where in the World you live and so on.
The best thing to do is to look at other 2nd-hand BALANCE saddles for sale on our website and elsewhere on the internet to get a rough idea of what sort of price similar BALANCE saddles are going for.
Some of our BALANCE registered Saddle Consultant do offer a service where they will do their best to help you to sell your saddle, in which case they should be able to give you a rough idea as to how much to ask for it.