The cost of what you are giving up when you make a decision involving scarce resources like time and money.
A full-time indie developer is giving up a steady salary to work on a game they are passionate about. That salary they will no longer receive is the opportunity cost. They are giving up the opportunity to earn that money by pursuing indie game development.
Game development is an exercise in making choices about how to use scarce resources. Do you scale back a game concept to keep development time short? Should you shift time away from development for marketing? Think about both the benefits and the opportunity costs (what you could gain if the resources were used for other things) to be an effective decision-maker. In fact, the entire long-tail strategy is based around opportunity cost, and leveraging optimized marketing efforts to reduce total development time, and have more time to develop follow-on projects.
War of Rights