There's a really useful ranking here: http://www.geteducated.com/online-college-ratings-and-rankings/best-buy-lists/best-buy-online-masters-mba-aacsb of AACSB-accredited MBAs which are available online. It's from late 2009, so check the figures. Only US schools are listed.

Interesting, but apparently they're ranked only by affordability - I clicked through the top ranked school - Southeast Missouri State University and although it received an "A" grade in cost, it got a "D" in student satisfaction.

Apparently, you pay for what you get.

Yes, the ranking suggests that it is be based on an overall score made up from: Affordability, Student Satisfaction and Public Perception. However, they are obviously are ranked by tuition fees only (specifically, they are ranked by in-state fees). It's certainly no kind of a rigourous study, but it probably does list almost all the AACSB online MBAs in the US with fees under $31,000.