We let people decide the value of most things (movies, products, etc.) because they are the ultimate users. Why not let the users have a say in the value of an idea? Prepare a form with the following questions and make it viral. Attach a small incentive to it to motivate more users. There are websites that pay users to answer survey questions. You can explore those, too.
The question you need to include:
Were you looking for a solution to the problem that the current idea solves?
Once you get responses, you can start analyzing:
Calculate the number of people interested in using your idea (indicated by "c" in the formula below).
Calculate the total number of responses you get (indicated by "pop" in the formula below).
Now, there are certain other parameters that the idea needs to fulfill. These come from experts in the field (As J. Nikola mentioned):
Necessity: The panel of experts rates the idea for its necessity in the current scenario. They give a score out of 1. A score of 1 indicates necessity and a score of zero indicates no necessity or that an alternative better solution exists. The average score is considered (indicated by "np" in the formula).
Conciseness: The panel of experts rates the idea for conciseness. They give a score out of 10. Extremely concise equals 1; extremely lengthy to the point that very few read it equals 10. The average score of the panel is considered.
Feasibility: Feasibility is scored in terms of the number of years it will take to realize (bring it to market) the idea. A score of 100 indicates that the idea is realizable within a year. For each extra year the idea takes to realize, the feasibility decreases exponentially.
Originality: Originality is binary, it can be either "yes" or "no". This is decided by the panel of experts. If the idea is original, it gets a score of one, else a zero.
Complexity: If the usage of the idea is too complex to understand and perform, few will be able to use it. Others might come up with a better and easy alternative to it. If the idea is least complex, the score is 1; if the idea is maximally complex, the score is 10.
Value of your idea (in $) = ((c + (np*pop)) * feasibility * originality) / (2 * pop * complexity * conciseness)
Therefore, if the idea is original, concise, least complex, feasible within a year, 100 out of 100 responders vote to use it, and the expert panel gives it a necessity score of 1,
The value of the idea is = ((100 + (1 * 100)) * 100 * 1) / (2 * 100 * 1 * 1) = $100.
This is the best score an idea can get. The score decreases hereafter for every fault.
Explaining the formula: The essence of an idea is that it needs to be new, not thought of before. If there is no originality, it may not be considered an idea, it is simply a useful product. Therefore, if the idea is not original, its value will be zero according to the above formula. Both, the expert panel and the consumer need to decide the necessity. Hence, necessity is the average of the score given by the panel and the number of people interested in using the idea.