便宜 (pián yi) 'cheap' joins 便 and 宜. 便 (read pián here) carries a sense of convenient or advantageous, and 宜 (yí) means fitting or suitable. Read together as pián yi the phrase came to mean low in price, a bargain. The same first character is read biàn in words like 方便 (convenient).

Origin

compound

Root

便 (pián, advantageous) + 宜 (yí, fitting)