That's a tough call. In some parts of the country that machine could still bring near new price because it performs well and they are getting hard to find.
Ballpark the mower sold for around $8500 when new and you can figure $5-$10 depreciation for every hour of use. In some cases it will be less in others it will be more.
Again this is only a ballpark estimate. The mower is really worth what ever you think it is worth and nothing more. I might think it's worth $6k but you might think it's worth more.
The Explorer was a great machine but time and technology passed it by. It would still be a great machine today just real slow in comparison to a Lazer. If you've got the bagging model be sure to adjust the belts per the operators manual. If the manuals have been lost e-mail me with the serial number and I'll have one sent out.
I would like to see that mower come back around. Did it have counter rotating blades and an impeller/vacuum system? Any plans for the re-production of this mower? What would it take to get it back into production?