sixty-four-dollar question

or $64 ques·tion

[ siks-tee-fawr-dol-er kwes-chuh n, -fohr- ]
/ ˈsɪks tiˌfɔrˈdɒl ər ˈkwɛs tʃən, -ˌfoʊr- /

noun

the critical or basic question or problem: Whether the measure will get through Congress this session or not is the sixty-four-dollar question.

Origin of sixty-four-dollar question

from the fact that 64 dollars was the largest prize on a popular radio quiz show in the 1940s