You didn't say if it was a Classic, Silver Eagle or what. I would say that mower is worth from 6500 to whatever more you can get. A Classic is their industrial mower. They call the Silver Eagle a commercial mower. A Silver Eagle and Golden Eagle are a step below a classic. The Xcaliber is their best mower.
I would say look on Ebay and Craigslist and see what a similar mower is going for. You probably have to sell this mower for an amount that would keep a person from going ahead and buying a new one. I think the warranty is based on time and or hours. I'm not sure if it is transferable. That will have to be considered also. I'm in the south so mowers here might sell for a little less than up north and in a bigger city. Go to the Dixie Chopper site. You can info on dealers in your area. Call them and get a price for your kind of mower.