despite; even though ~
Expresses that something is done or happens despite a prior effort or expectation, emphasizing that the effort was wasted or the result is contrary to what was hoped.
甲斐もなく is a formal, written expression meaning 'despite (the effort/expectation)' or 'in vain'. It highlights that the outcome is disappointing or meaningless compared to the effort put in. The 甲斐 (worth, effect) is negated with もなく, so it literally means 'without any worth/effect'.
This pattern is typically used in narratives, essays, or formal speech, not casual conversation. It often follows a past tense verb that describes the effort or action that turned out to be futile, or a noun phrase indicating the expected benefit. The sentence usually describes a negative or ironic result.
Because it is a fairly literary pattern, using it in everyday chat might sound overly dramatic. But in writing or formal contexts, it adds a nuanced tone of regret or resignation.