Compensation at higher levels is a lot beyond reported gross salaries. If I were to calculate ROI based on a linear progression, I should not even be bothering to do MBA because of the so called "opportunity costs"
A few things are intangible - like the experience, alumni network, and simple the fact that we may want to do it for the knowledge. One of the other factors that is completely missing from ROI calculations is "opportunities." How would you calculate payback of starting a successful business with a few good classmates? How would you calculate returns on being able to do well in your business, marketing or account management job because you had good contacts from your alumni in higher positions in your target companies?
A lot of monetary calculations are hidden - and some are discontinuities (stock grants, profit payouts, bonuses, perks, chances to work in tax-free regions and so on). If we were all to look at a good MBA returns purely based of annual gross salary increases, it would not make sense to a lot of us.