1. What does it mean?
멍청비용 combines foolish with cost. People use it self-deprecatingly for money lost through an oversight, such as buying the wrong size or missing a cancellation deadline.
Also written or said as meongcheongbiyong
2. The nuance
It can be cruel when applied to another person's hardship. Not every unexpected cost is avoidable, and the phrase should not be used to blame victims of deception.
3. Examples
“날짜를 잘못 봐서 변경 수수료라는 멍청비용이 생겼어.”
I read the date wrong and ended up paying a change fee for my mistake.
“같은 물건을 두 번 주문한 건 이번 달 멍청비용이네.”
Ordering the same item twice was this month's avoidable expense.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.