Searching is inherently an interactive process usually requiring numerous iterations of querying and assessing in order to find the desired amount of relevant information. Essentially, the search process can be viewed as a combination of inputs (queries and assessments) which are used to "produce" output (relevance). Under this view, it is possible to adapt microeconomic theory to analyze and understand the dynamics of Interactive Information Retrieval. In this paper, we treat the search process as an economics problem and conduct extensive simulations on TREC test collections analyzing various combinations of inputs in the "production" of relevance. The analysis reveals that the total Cumulative Gain obtained during the course of a search session is functionally related to querying and assessing. Furthermore, this relationship can be characterized mathematically by the Cobbs-Douglas production function. Subsequent analysis using cost models, that are grounded using cognitive load as the cost, reveals which search strategies minimize the cost of interaction for a given level of output. This paper demonstrates how economics can be applied to formally model the search process. This development establishes the theoretical foundations of Interactive Information Retrieval, providing numerous directions for empirical experimentation that are motivated directly from theory.