Economists suggest that the Catholic Church has been losing adherents not because people stopped believing in God but because membership became too cheap compared with evangelical Christianity, which demands a bigger investment in its churches from members and thus inspire more loyalty.
From The Price of Everything by Eduardo Porter. Actually this explanation is not necessarily 'rational' because if the investment upfront is sunk it is not obvious why it should have a bearing on church attendance, unless the expected return is also high. Explaining it purely on the cost side suggests some irrationality on the part of the attendants. One possibility of course is that "price of attendance" may act as a screening out device (i.e. evangelical churches only getting committed people) but that would only explain why evangelical churches may be stable not necessarily increase over time!