Old Mason jars are popular collectibles and can sell for a lot of money if they are rare or in high demand. Best value can be found in unusual colors like purple (amethyst), yellow, or olive green. Beware of very weird colors like hot pink which would indicate a reproduction. Value also can be found in jars with complicated or unusual closures, upside down lettering, or odd shapes. Before you can figure on a value you need to identify the type of jar that you have. Learn the date, the style, color, and closure. You can identify your specific jar by checking out some books including:
"One Thousand Fruit Jars" by Bill Schroder
"Collectors Guide to Ball Jars" by William F, Brantley
"Collectors Guide to Old Fruit Jars" also called "Redbook #9"
In addition, there are many online sites that can help you learn more, some dedicated to glass products made by Hazel Atlas. You can also check out the Federation of Historical Bottle Collectors online. They provide a wealth of information. Do not rely on older books for values.
After you have identified your jars, look at sold prices at online auctions. I recently saw a quart jar labeled "Ball Perfect Mason" in green that sold for $50.00. Very unusual or high demand jars can be priced in the hundreds of dollars.