Mostly anecdotal, which is why I said "suspects". I'd love to see a comprehensive comparison of university endowment growth over the last few decades. I suspect the data's all there, but nobody's pulled it together. I've come across a number of articles discussing large increases in endowment size and a number of universities of late, seems to be a common theme as more and more people are starting to ask why college has become so expensive. For example there was recently a story in the Washington Post about Washington and Lee University which covered how it had gone in the 90s from a school with a relatively paltry endowment to land in the top 30 overall.

These two sources are interesting:

http://www.jbhe.com/features/57_pellgrants.html

http://en.wikipedia.org/wiki/List_of_colleges_and_universiti...

The first source leaves little question that it's happening at top tier universities, but I'm interested in the trend and second- and third- tier schools.

edit: As another commenter points out, most schools aren't sitting on large endowments. There probably isn't a single unified cause to explain rising costs. Endowments are probably part of the story for higher-profile schools, but not so much for others.