All languages have exceptions alongside overarching rules and regularities. How does a young child tease them apart within just a few years of language acquisition? Drawing an economic analogy, Yang argues that just as the price of goods is determined by the balance between supply and demand, the price of linguistic productivity arises from the quantitative considerations of rules and exceptions. The learner postulates a productive rule only if it results in a more efficient organization of language, with the number of exception falling below a critical threshold. Supported by a wide range of cases with corpus evidence, the Tolerance Principle gives a unified account of many long-standing puzzles in linguistics and psychology, including why children effortlessly acquire linguistic rules that perplex otherwise capable adults. The focus on computational efficiency provides novel insight on how language interacts with the other components of cognition, and how the ability for language might have emerged during the course of human evolution.