Sunk cost is a term that is used in economics to refer to a cost that has already been incurred and cannot be recovered. It is a cost that has already been "sunk" into a project or investment, and it is not relevant to future decision-making. Sunk costs are different from future costs, which are costs that have not yet been incurred but will be incurred in the future.
One of the key principles of economics is that decision-making should be based on marginal costs and marginal benefits. In other words, when making a decision about whether to invest in a project or not, the relevant costs and benefits are those that will be incurred or received as a result of the investment, rather than those that have already been incurred. Sunk costs are therefore not relevant to this decision-making process.
However, it is common for people to fall into the sunk cost fallacy, which is the tendency to continue investing in a project or activity simply because they have already invested a lot of time, money, or effort into it. This can lead to irrational decision-making, as the decision to continue investing in the project is based on sunk costs rather than on the marginal costs and benefits of the investment.
For example, imagine that you have invested $500 in a concert ticket, but on the day of the concert, you become sick and are unable to attend. The $500 that you have already spent on the ticket is a sunk cost, and it should not influence your decision about whether to sell the ticket or not. The only relevant consideration is the marginal benefit that you would receive from selling the ticket, which would be the amount of money that you could get for it.
In economics, sunk costs are often contrasted with opportunity costs, which are the potential benefits that are foregone as a result of making a particular decision. For example, if you decide to attend the concert, you are giving up the opportunity to do something else with your time and money. This opportunity cost is relevant to your decision, as it represents the marginal benefit that you would have received from the alternative activity.
In conclusion, sunk costs are costs that have already been incurred and cannot be recovered. They are not relevant to future decision-making, and it is important to avoid falling into the sunk cost fallacy by basing decisions on marginal costs and benefits rather than on sunk costs.