For the budget, you're not calculating the entire equation, mate.
You're forgetting how much the time of an employee costs. Let's assume a single user license for Office costs $100 dollars (I'm not sure how much it costs, but I think that's ballpark). Now, if your employee is used to Office Word, and using the more streamlined MS Office provides him with a 0.5% increase in productivity over Open Office, how much money is that each year??
Lets give this guy a yearly salary of $45,000 and calculate 0.5 * 0.01 * 45000 = 225. => 0.5% of his work time is worth $225 each year.
Oh, wow, that MS Office package just paid for itself in less than six months! and that's by estimating only 0.5% work increase. And trust me, a lot of people spend their entire work day using Excel, and having used both Excel and Calc I can GUARANTEE you the increase in productivity is much, much higher than half a percent (for perspective, 0.5% of an 8 hour workday is 2.4 minutes)
Don't forget get this when you're arguing about how to spend taxpayer money. There's a reason why big companies / institutions pay for expensive software; it's actually cheaper in the long run!