the sixty-four thousand dollar question something that is not known and on which a great deal depends.
☞ This expression dates from the 1940s and was originally the sixty-four dollar question, from a question posed for the top prize in a broadcast US quiz show. It has been hit by inflation, and much larger hypothetical prizes are now common, such as the million dollar question and even the trillion dollar question.
2010TechDirt When you think about it, why would God want people not to work on Sunday? Now that is the 64-thousand dollar question.