noun
wasted money; money down the drain
Refers to money spent without any benefit or return; often used in expressions of regret or criticism.
そんな安物を買うのは無駄金だ。
Buying such cheap stuff is a waste of money.
高い会費を払ったが、結局無駄金になった。
I paid a high membership fee, but it turned out to be money down the drain.
無駄遣い is the act of wasting money, while 無駄金 is the wasted money itself.
Compound of 無駄 (muda, 'waste') and 金 (kane, 'money'), with rendaku voicing of kane to gane.