Sunk cost fallacy
Continuing to invest in a decision because so much has already gone into it: past spending does not come back because new spending is added to it.
How to read the number
The right question is always the same — would I buy this today at this price if I held nothing. A no means that holding on is a purchase you would not make.
When the metric lies
The error disguises itself as loyalty to a plan. The difference is simple: a plan describes the exit conditions in advance, while sunk costs are recalled instead of them and after the fact.
Also known as: throwing good money after bad