A money book may describe a decision made worse by a payment that cannot be recovered. The useful question is which costs still change between the options available now.
Separate the old payment from the next choice
The St. Louis Fed’s explanation of economic thinking introduces sunk costs as past costs that cannot be recovered. This is a way to structure a comparison, not a claim that regret disappears once the arithmetic is clear.
Imagine a fictional workshop ticket that cannot be refunded. Attending would now require another journey and several hours; not attending would leave the ticket unused. The ticket payment is the same under both remaining options.
Check whether the cost is truly unrecoverable
A refundable booking is a different case. Read the actual terms rather than assuming every past payment is sunk. Future charges and cancellation costs may also change the comparison.
In the margin of the book, draw two columns: already fixed and still affected by the decision. This helps you evaluate the author’s example without turning it into a blanket instruction to abandon any project that becomes difficult. The relevant comparison starts with the choices that genuinely remain.