Well when we say 'that's a million dollar question' it's not just an important question but THE most pertinent question. The question that begs for an answer. The question that represents the highest curiosity. This means a very difficult question to find the answer for and a question so difficult that whoever answers it gets a million dollars.
In any problem solving situation or mystery, It is necessary to take all the available information and use it to realize the actual problem. This problem definition is referred to as the million dollar question and is often associated with an "aha moment"