if youre selling i guess its worth what you can get some muppit to pay pay for it.... if youre using it, its worth what it would cost to get something else to do the same job
try looking at tractors for sale on ebay...
theres a monthly mag on sale in our tesco , tractor and machinery , which has ads and reports of what stuff has made at recent auctions, there are other old tractor mags there aswell which may do the same but i cant remember the titles.
when i got our 6711, i didnt need or want it , the hippy traveler was desperate to get shot of it, cost me an old bike, an old milk churn and the 60 i had in my pocket at the time.