500-yen bento; one-coin bento
A boxed lunch sold for a single coin, typically a 500-yen coin. Common in convenience stores and supermarkets.
I bought a 500-yen bento at the convenience store.
Recently, the variety of 500-yen bentos has been increasing.
Compound of ワンコイン (from English 'one coin') and 弁当 (bento, boxed lunch).