How much is a 1978 Marvel Comics group the avengers worth?

Comic book prices vary greatly depending on the desirably of the issue in question, its condition, and its rarity.

For a quick 'snapshot' of potential work, search eBay for auctions of the issue. Be sure to look for actual sales not just listings since some sellers might inflate their prices. Note that condition plays a big part: a "near mint" comic may fetch 10 times more than one only in "good" condition.

Otherwise, the Overstreet Comic Book Guide is widely considered to be the authority on comic book prices and grading.