The optimal mechanism for selling a divisible good under convex production costs can be arbitrarily complex, yet firms overwhelmingly use two-part tariffs: a fixed fee plus a constant markup over production cost. We quantify the profit this simplicity sacrifices. For regular value distributions, a two-part tariff guarantees a fraction of the optimal profit that depends only on a lower bound m on the elasticity of the marginal cost, independent of the value distribution. The guarantee approaches 1 as m grows, and is impossible without such a bound. Beyond regularity, no two-part tariff guarantees a constant fraction, but a menu of K + 1 tariffs with a common markup does when the ironed virtual value has K ironing intervals; moreover, the menu size must scale with K. With multiple product lines, the better of separate sales and a single access fee granting purchases at production cost achieves a constant fraction of the optimal profit. These results provide a theoretical foundation for the ubiquity of simple cost-based pricing.