Prices vary around the country. A pristine, running, 5-1/2 in Minnesota may go for $200-$300.

But I would not pay over $150 if it hasn’t been maintained and run. If it hasn’t been run since the 1950s or 60s, it will need approx $150 of parts that go bad with age whether it’s been run or not. You can count on coils, and impeller being bad and probably lower unit seals, gas tank diaphragm and dual gasline being bad.
Dave