There's a whole lot of things to look at when you buy a 22 year old bike. We probably don't need to talk about the obvious stuff like frame, suspensions, brakes...
Check the (original?) tank for internal corrosion.
Make sure if the bike has the original engine (which can be tricky on old Jap bikes) or if it was replaced by a later XT engine.
All the earlier 600cc Yammies have slightly undersized seats for the main bearings in the engine casing. Very difficult to fix (complicated machining or replace crankcase).
Prices vary widely depending on the condition, mileage as well as the country where you buy.
You would probably battle to get more than 1000 Euro for the bike in good condition in Europe whilst a 30000 km good running condition one would easily fetch more than 20000 Rand (2800 Euro) here in South Africa.