Opportunity cost is a key concept in economics because it implies the choice between desirable, yet mutually exclusive results. It has been described as expressing "the basic relationship between scarcity and choice. The notion of opportunity cost plays a crucial part in ensuring that scarce resources are used efficiently. Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, pleasure or any other benefit that provides utility should also be considered. There is always an opportunity cost in a decision that is made either in economics or everyday life.
A person who invests $10,000 in a stock denies themselves the interest they could have earned by leaving the $10,000 dollars in a bank account instead. The opportunity cost of the decision to invest in stock is the value of the interest.
An organization that invests $1 million in acquiring a new asset instead of spending that money on maintaining their existing asset portfolio incurs the increased risk of failure of their existing assets. The opportunity cost of the decision to acquire a new asset is the financial security that comes from spending the money on maintaining their existing asset portfolio.
If a city decides to build a hospital on vacant land it owns, the opportunity cost is the value of the benefits forgone of the next best thing which, might have been done with the land and construction funds instead. In building the hospital, the city has forgone the opportunity to build a sports center on that land, or a parking lot, or the ability to sell the land to reduce the city's debt, since those uses tend to be mutually exclusive. Also included in the opportunity cost would be what investments or purchases the private sector would have voluntarily made if it were not taxed to build the hospital. The total opportunity costs of such an action can never be known with certainty (and are sometimes called "hidden costs" or "hidden losses", what has been prevented from being produced cannot be seen or known). Even the possibility of inaction is a lost opportunity (in this example, to preserve the scenery as-is for neighboring areas, perhaps including areas that it itself owns).
It can also apply to time; one might use a limited vacation time to travel to a place of cultural enrichment or to do household improvements. Thus the two-week road trip might preclude repairing or painting one's house that year.
In real life an opportunity cost is an ongoing process of people believing that they are losing or gaining the efficiency in certain decision making. This can be related more to psychological aspect either to economical. The reason for that is nobody can predict reality exactly.
Opportunity cost is not only assessed in monetary terms, but can be assessed in terms of anything which is of value. For example, a person who desires to watch each of two television programs being broadcast simultaneously, and does not have the means to record a channel other than the one being viewed, can only watch one of the desired programs. Therefore, the opportunity cost of watching Dallas could be missing Dynasty. In a restaurant situation, the opportunity cost of eating steak could be not eating the salmon. The opportunity cost for the diner of ordering both meals could be twofold - being $20 poorer (a bigger financial cost), and being thought gluttonous or extravagant by his peers (a social or interpersonal cost).
The consideration of opportunity costs is one of the key differences between the concepts of economic cost and accounting cost. Assessing opportunity costs is fundamental to assessing the true cost of any course of action. In the case where there is no explicit accounting or monetary cost (price) attached to a course of action, ignoring opportunity costs may produce the illusion that its benefits cost nothing at all. The unseen opportunity costs then become the implicit hidden costs of that course of action.
Note that opportunity cost is not the sum of the available alternatives when those alternatives are, in turn, mutually exclusive to each other. The opportunity cost of the city's decision to build the hospital on its vacant land is the loss of the land for a sporting center, or the inability to use the land for a parking lot, or the money which could have been made from selling the land, as use for any one of those purposes would preclude the possibility to implement any of the others.
However, most opportunities are difficult to compare. Opportunity cost has been seen as the foundation of the marginal theory of value as well as the theory of time and money.
In some cases it may be possible to have more of everything by making different choices; for instance, when an economy is within its production possibility frontier. In microeconomic models this is unusual, because individuals are assumed to maximise utility, but it is a feature of Keynesian macroeconomics. In these circumstances opportunity cost is a less useful concept.
A health example can be found from the Baltic herring, which has been contaminated with dioxin from its environment, to exceed European Union limits. The most obvious response would be to ban it, but Finnish officials have not decided to do so. The reason is that the opportunity cost is that the person would not eat the fish and get a deficiency in omega 3 fatty acids. The opportunity cost of the deficiency is higher than that of theoretical risk from dioxin toxicity.