贝 on the left is the shell radical, an ancient cowrie used as currency. It tags 贬 as a value word, grouping it with 财 (wealth), 贵 (expensive), and 贱 (cheap) - all turning on how much something is worth.
乏 supplies a phonetic that has drifted from fá to biǎn along an old labial pair. The lacking sense also colors the meaning: to 贬 is to mark something as deficient, demote it, declare its worth fallen short. Compare 泛 (overflow).
