Opportunity cost is a key concept in economics to express the value one misses out on when choosing one alternative over another. This concept is used to explain rational decision making in a scenario where multiple mutually exclusive alternative choices can be made. In this paper, we explore this concept in the realm of open-source software. We look at the different ways for measuring the cost and these can be used to support decisions involving open-source software. We review literature on opportunity cost use in decision support in software development process. We explain how the opportunity cost analysis in the realm of open-source software can be used for supporting architectural decisions within software projects. We demonstrate that different measures of costs can be used to mitigate problems (and maintenance complexity) arising from the use of open source software, allowing for better planning of both closed-source commercial and open-source community projects alike.