How much is an envelope from World War 2 worth?

The variables that affect this are too many to allow any sort of guess.

From what country to what destination?

Soldier or civilian?

Censored or non-censored?

What stamps are affixed?

What is the cancellation?

It could be anything from what you could find in a $1 box to many thousands of dollars for s specific Field Post Office, or location, or even a specific date/time stamp. I once saw an envelope with a postmark of Pearl Harbor, Hawaii, at 7:59am on Sunday, December 7th, 1941 available for $500.