E-books have an important place in library collections in supporting education and research activities. Libraries need to follow and evaluate different acquisition models which vary by supplier, in order to create an effective and balanced collection by using their budgets efficiently. There are different e-book acquisition models that show under which conditions content by suppliers may be accessed. These models are constantly changing due to economic conditions and technological developments. The purpose of this study is to investigate how the choice of acquisition model affects total library expenditures, whether the choice of model changes according to the discipline, and which are the most cost-effective unit-use procurement models. In this study, based on real usage data from Hacettepe University Libraries, e-book acquisition models offered by the publisher and aggregator were tested and, then cost and subject analyses were made with the developed scenarios. Among the models offered by the publisher a unit usage cost of 1.18(sic) was seen in the EBA (Evidence Based Acquisition) model. If this model was preferred there would be guaranteed lower expenses. Among the models offered by the aggregator, the lowest unit usage cost ($13.75) was seen in the DDA (Demand Driven Acquisition) model with STL (STL - Short Term Loan). With the use of STL, the most used titles were purchased automatically, and the DDA model with STL became an economical option for the library. It was concluded that the frequency of e-book usage varies according to discipline, and unit usage costs vary depending on the total cost paid and the number of uses. The budget of the library, the supplier conditions, and usage data affect the preference of acquisition models according to discipline. The results of this research, in which a comparative cost analysis of e-book acquisition models was performed, should be used in collection development and management. Usage data should be analyzed, and the most appropriate and economic acquisition models should be determined, in order for budgets to be used more effectively.