Why does a magnum of champagne equivalent in volume to 2 standard bottles cost more than twice the price of a single bottle?

SNOB VALUE

Actually, there are a couple of reasons. First, magnum bottles are, in general, less common and therefore slightly more expensive to produce. Second, some (not all) magnums are aged longer before release. Third, it is generally believed that wine ages best in a magnum (there is a lower air/wine ratio) and therefore they are generally more desirable for connoisseurs/collectors who are willing to pay more for the added benefit.