What is the value of a blue mason jar?

This question is rather broad. The answer depends on many factors. Size, age, condition, factory, etc. Vintage, Quart, Ball jars are in more demand than say Pint, newer, Atlas jars.

These jars also have a tendancy, not only to fluctuate with season, for canning if selling newer jars. But also have an up and down history. Some years these sell like hotcakes and you can't sell enough, then..bang, just like a faucet turning off. No one wants one. Very volatile market.

The darker the blue the better. Better condition, Ball jars in pint size are selling on average of 5.00 ea, and quart size 10.00 ea.

Hope this helps!