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In a competitive market, why is movie theater popcorn expensive?
While many answers correctly cite the effect of monopoly power.... there is a cost issue that no one has yet posted.  I first recall seeing this cost effect in a managerial econ textbook, perhaps Ivan Png's.   The theater must clean up the popcorn mess.  The sales of popcorn elsewhere does not usually include the costs of disposal because that cost is not borne by the vendor.   In a theater the cost of disposal -- which is a variable cost depending on the amount and type of foodstuff sold -- is borne by the vendor, who must pay employees to clean up between shows and at the end of the night.  While most people are responsible with popcorn, there is a long tail of more and more costly messes left by the customer...  and if the theater shirks the cost of cleaning the messes then rats with long tails will bite into future customer demand for tickets. Whether this cost effect is as large as the monopoly power effect and the synergy in willingness-to-pay between entertainment and refreshments is not clear.  All of these effects may be in operation.