I think Night Glider tends to go between 20 and 30 dollars, depending on condition.
As for MLPs in general:
Most of the (G1) MLPs I see don't go over $20, and very many of them stay under $10. However, that's a
very general statement. Every pony is different. There are plenty of rare or desirable ponies that go far above that. If the pony is in exceptionally nice condition that can make them expensive, too.
You can always post a Price Check thread (PC) here asking what a good price would be for something, and someone will let you know.