There is no rule of thumb as it all depends on the idea, and the effort and money you put in to turn an idea into a business or product and how profitable it would be.
For example, if you had an idea for a space shuttle it would take billions to implement, and unlikely to be profitable. What was the idea worth? But if it was an idea a new computer networking structure, and you spent a day to refine it and turn it into business plan, and the next day brought it to a Venture Capital firm who invested millions, that's something else.
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