Cost a bomb

British slang using "a bomb" as shorthand for a large sum of money, in the same family as related phrases like "make a bomb." Exactly why an explosive became slang for a large quantity of cash isn't documented anywhere, though it may simply echo the sense of sudden, forceful impact, a bomb going off, applied figuratively to a price that hits hard. The phrase is well attested in British use from the mid-20th century, without a confirmed origin story behind the choice of word.