Don't get me wrong, migrating away from any proprietary application can be very time consuming and costly. However, in the big picture, migrating to open source software will save in the long run. The migration is a one-time move, after that move is done no more upgrades and licenses need to be purchased ever again. There are also many many other bonuses to using open source software that will add up over time. Open source software is free as in freedom, and is developed by the community, not a single entity behind closed doors.
Do you run a business that involves any form of computer rental such as internet cafe, for-profit computer training center, and/or hotel that provides computers for the convenience of the guests? Do you live in a country like Taiwan where BSA (Business Software Alliance) does yearly raid to small and medium businesses with the help of your government because they are pressured by, or have some inconvenient relationship with BSA? My guess is that BSA will target this new group of software "pirates" -- yes even if they have already paid for the first layer of the Microsoft taxes. But there is a way -- maybe even a much more profitable way -- out if you are willing to try diskless computers + mother tongue bootable usb keys.