A lot depends upon maintenance history.
How does the 20-year-old bike run?
A typical aging syndrome is: Petcock gasket deterioration. Crud gets into the carburetor and honks things up when this phenomenon occurs. Pox may be present, maybe not--could have been previously addressed.
History remains critical . . . was bike in STORAGE much of this time? How recently has it been run? Are all fluids fresh?
All this said, . . . I think the bike would have to be in rather exceptional, if not concours (spell-checker says I spelled the last word wrong), condition, to demand the price, given alternatives. Maybe not. See if a KLR650 of that vintage is listed on Kelley Blue Book for your area, for some vague and loose guidance.
Otherwise, even a KLR650 that old, technology-wise, has a lot in common with a 2017 model; a 2007 model, anyway!