Modeling how people interact with search interfaces has been of particular interest and importance to the field of Interactive Information Retrieval. Recently, there has been a move to developing formal models of the interaction between the user and the system, whether it be to: (i) run a simulation, (ii) conduct an economic analysis, (iii) measure system performance, or (iv) simply to better understand the interactions and hypothesise about user behaviours. In such models, they consider the costs and the benefits that arise through the interaction with the interface/system and the information surfaced during the course of interaction. In this full day tutorial, we will focus on describing a series of cost (and benefit) models that have been proposed in the literature and how they have been applied in various scenarios. The tutorial will be structured into two parts. First, we will provide an overview of Decision Theory and Cost-Benefit Analysis techniques, and how they can and have be applied to a variety of Interactive Information Retrieval scenarios. For example, how do we reduce the cost browsing facets? how many results should we show per page? and how do we get users to pose longer queries? The second part of the day will be dedicated to building cost models where we will discuss different techniques to build and develop models from which one can draw testable hypotheses from. In this more practical session, we will also discuss how costs and benefits can be estimated, and how the models can help inform and guide experimentation. During the tutorial participants will be challenged to build cost models for a number of problems (or even bring their own problems to solve). We will then conclude with presentations of solutions followed by a summary and overview of challenges and future directions. This tutorial is aimed at participants wanting to know more about cost models and how they can be applied, interpreted and built in the context of information seeking, search and retrieval.