How does one determine how much to get paid in the creative industry like Movies/Music where the success of the output is abstract and not measurable?

You have given two examples of AR Rehaman whom every Indian is aware of, but your are talking about Santosh - we people of out side Tamilnadu are not aware of.  Creative industry - whether it is advertising or acting or singing or music direction etc  including the branding tactics etc - is such which requires major push or major hit so that or say certain time to get the real value.  You may feel that you are very much creative but your way of singing or your copy writing or your branding ideas need to be recognized at a level or need some hit without which you have no scope of determining your price.  I may feel that my design or singing etc worth few hundred thousands but the man who gave me first opportunity is more valuable to me rather than the money he has paid or wanted to pay. I need more opportunities more exposure than the money I wanted.  In fact once I have been recognized I can demand but not now. A producer or an agency or a client decides based on the weight of his heart and purse.  In fact he do not want you ( provided your work is excellent and truly creative) to be recognized, so he wants you to feel that you got that money as a donation so you will come back to him for next assignment.