$64,000 question, the$64,000 ques·tion, the /ˌsɪkstifɔː ˌθaʊzənd ˌdɒlə ˈkwestʃən $ -fɔːr-, -ˌdɑːlər-/ noun 
the most important thing that people want to know about something: 
The $64,000 question is whether or not the rocket will take off safely.
the most important thing that people want to know about something: The $64,000 question is whether or not the rocket will take off safely.