While the following is related more to "open access" publishing (a subset of the 'open' movement but arguably not the same as OER) I think it still really important - http://jisc.ac.uk/news/stories/2009/01/houghton.aspx is reporting that a study in the UK finds that in addition to the benefits to the public of having open access to knowledge,
Now those are the kinds of numbers to get people's attention. So while the playing field is slightly different, I'd warrant the effect would be similar with OERs.
"When considering costs per journal article, Houghton et al. believe that the UK higher education sector could have saved around £80 million a year by shifting from toll access to open access publishing. They also claim that £115 million could be saved by moving from toll access to open access self-archiving."