At what school ranking is a MBA not worth it?

I think you're making some assumptions that don't necessarily apply for everyone. You're assuming that everyone has a job that they want to stay at, with reasonable chances of advancement, and that ROI is the only reason to get an MBA.

Some people get an MBA because they want to change careers; business school is a great way to do this. Some people are at jobs where the chances for advancement are not necessarily linear; sometimes people are at jobs where advancement is only possible with additional education. And ROI isn't the only reason to get an MBA. Some of my friends are looking for jobs at non-profits. They aren't going to make a bunch of money (at least that's not the primary goal), but they wouldn't get these types of non-profit jobs without an MBA.

But if you're looking for just plain career advancement and ROI, elite & ultra-elite (top 15) are no-brainers. Beyond that, you need to do your research and analyze your expenses and expected returns.