It really depends on the individual. There isn't a rule of thumb that applies to everybody. For others, it may not be worth going back to school at all, even if it were a top school.
Put it this way. Each person goes to b-school for different reasons. Some go strictly for more money and promotions within their pre-MBA industry. Others go to switch careers. And yet others go because they're young and bored. Some go as a sabbatical from their career, money/ROI be damned. Some go for ego reasons. Others go because they want some branding on their resume. It's different for every person.
Also, whether it's worth it also depends on the person's career and life situation -- some folks have very strong blue chip resumes, where branding isn't important. Others may already have a family, which makes the decision to go back full-time, part-time or not at all a more complicated one, let alone deciding what "tier" of school to target.
Again, you can't really generalize. What matters most to YOU is your own individual circumstance and situation - and what kind of MBA (if any) is worth it to YOU.