If you really want to compute this, I think you'll need to use marginal benefits of math skills, which varies a lot by person and environment. It depends on not just other aptitudes, but also preferences and circumstances surrounding that person. Britney Spears or Tiger Woods probably wouldn't benefit much from Calculus.

Even for an 'average' salary person, it depends a lot on other skills s/he has. A top salesperson/marketing wizard does not need calculus and can earn several times the salary of the best engineer who does not pursue business or get involved in a startup.

The premise that Q-N is positive could be flawed from the start. See, e.g. "A Whole New Mind" http://www.amazon.com/Whole-New-Mind-Information-Conceptual/...