That productivity decrease will only take place while your employee is getting to know the software. After a few months it should be about the same, but you still need to pay the license fees every so often.

In this particular case most of the employees have been working exclusively on OpenOffice for the last 4-5 years. Switching them to MS Office will probably decrease their productivity on top of the cost of the license.

Anyway, they didn't even try to claim that it will save them money. What they claimed was that 20% of their word processing cannot be done with open source tools. I am really curious what kind of word processing they do.