A long time ago, I took a few economics courses in college. I wasn't much good at them, but one thing I remember is the concept of "opportunity cost."
A person who has $15 can either buy a CD or a shirt. If he buys the shirt the opportunity cost is the CD and if he buys the CD the opportunity cost is the shirt. If there are more choices than two, the opportunity cost is still only one item, never all of them.
Anyway, I realized that this concept is really just common sense and that old Buddhist masters already understood the idea.
In Buddhism, the goal is spiritual progress for oneself and spiritual and material progress for others. In order to make spiritual progress in Buddhism, we need to use finite resources: our time, energy, money, etc. One of the Four Thoughts Which Turn the Mind to Dharma is the preciousness of human life and another is impermanence. Put together, these emphasize how the time we have in our present lifetime is a limited resource of an unknown total quantity, and it's important not to waste it. The opportunity cost of wasting a moment on something trivial, is the meaningful spiritual activity we could have been doing in that moment.
Some other evidence that ancient Buddhist masters understood the idea of opportunity cost is the 30th vow of the secondary Bodhisattva Vows which is to avoid excessive involvement in non-Buddhist subjects. Time spent on non-Buddhist things is time which could have been used for Buddhist things.
In any case, Buddhists seem quite aware of the finiteness of resources in their lifetimes, and they also see opportunity costs in terms of what they could have given to others: each bit of food or money which I consume now is an amount which I cannot later give to others instead (I believe the Jewel Ornament of Liberation has some quotes like this).
Now that the thought of one day doing a three-year retreat has arisen in my mind, I realise that I need to amass the resources necessary to do it: money, spiritual experience, knowledge, etc. This changes my perspective. Every penny I spend today is a penny which I will not be able to save for a three-year retreat. I won't let this turn me into a miser because it's still very valuable to give to others and to attend shorter Buddhist teachings in the meantime, but this perspective has changed my view of my actions in my life.
Anyway, I've rambled on long enough, and now the opportunity cost of continuing to write this is too high: I will now switch to meditating instead before I have to go to sleep!