Saying

[iwbus]

Why do things cost a bomb?

[Quizmonster]

The use of 'bomb' in such slang phrases was originally an American usage. Strangely, a very successful Broadway play was often called 'a bomb', presumably on the basis that it had taken off 'explosively', as it were, and made a lot of money for the producers. (On the other hand, used as a verb, 'to bomb' meant a play was a total failure!)
Whatever, 'bomb' has meant a large amount of money since the 1950s.