Miner extractable value (MEV) refers to any excess value that a transaction validator can realize by manipulating the ordering of transactions. In this work, we introduce a simple theoretical definition of the 'cost of MEV', prove some basic properties, and show that the definition is useful via a number of examples. In a variety of settings, this definition is related to the 'smoothness' of a function over the symmetric group. From this definition and some basic observations, we recover a number of results from the literature.