Money Rules to Live By VI

Throw no good money after bad
"Sunk costs" are expenses that have already been incurred and can't be recovered to any appreciable extent. "Sunk cost fallacy" means an irrational belief that a further investment of time, money or effort will somehow resurrect the value that's already disappeared.

A classic example is the investor whose stock has plunged because the prospects of the company have worsened. The investor wouldn't buy the same stock today, yet continues to hang on to the shares rather than sell them and take the loss. The investor may offer the excuse that he or she wants to at least "break even" before selling, but of course the stock market doesn't care about the investor getting the money back, and all the wishing in the world won't bring the stock price back up.
By hanging on to the shares, the investor is giving up the opportunity to invest elsewhere at a profit -- an opportunity cost.
Remember: Sometimes it is better to flee and hide, so that another day you may come back and fight.