I'm currently a 3rd year manager at a Big 4 firm and am going to bschool this fall. As you mention in your note, manager compensation at a Big 4 firm is more than the avg starting salary of an MBA, so the ROI becomes difficult on purely monetary consideration. Unless you go into a highly compensated industry post MBA (the alphabet soup of IB, HF, PE, VC, or MC), there's a good chance your post-MBA compensation - even over a longer time period - will not be much more than what you could have earned had you stayed at the CPA firm and become a partner. So basically what I'm saying is that purely looking at numbers, my ROI for an MBA was awful. Using avg. starting salary for the role I want post-MBA and reasonable estimates for future increases, I think my breakeven year came out somewhere around infinity.
But that's not the point. If I wanted to work for a Big 4 firm and become a partner, an MBA would be the last thing I did - unless it was an EMBA that the firm paid for. The main reason I decided to pursue an MBA is because the thought of working for a Big 4 firm or doing accounting for the next 35 years brought tears to my eyes. The MBA gives you pretty close to a fresh start, even in this economy, and gives you career options that no amount of Big 4 experience or a CPA can match. So I made the decision that, in the short run, I would be willing to give up potentially tens of thousands of dollars a year in order to pursue a career that I actually found rewarding.