Ninja 250 Value Calculator
Heres a value tool you can use. 20k miles on a 1994 says 600$ in good condition (no damage).
If the bike has 20k miles, that means it just passed over a scheduled maintenance period.... Refer to this page for the maintenance sched, and make sure the owner had it done.
Service Schedule - Ninja250Wiki
All the things on that chart should have been addressed.
Also, refer to this page for a full guide on how to assess a used motorcycle.
Used Motorcycle Evaluation Guide
Looking forward to helping you achieve the same fuel economy i am seeing with my Ninja 250!