This article suggests that if a government (in this case the Danish), moves over to Open Office as their standard application and resulting in ODF documents, there will be substantial savings, but that, if they did not move and kept to what they have (MS office I assume) and used the plugins from MS to create the MS-backed Office Open XML (now under an active ECMA standards track), then the govt would save more.

The Danish Open Source Business Association commissioned a report to do the numbers behind this effort. I think report is trying to be fair and does state that there are aspects that they are not considering because of the lack of valid data points. Need time to read it to come to my own conclusions, so watch this space!