According to Gartner, Google Apps for Business represents .5% of Google's almost 40 billion revenue. That would be about US$ 200 million a year. What are Microsoft's BPOS/365 revenue numbers?
I have no idea about the numbers but by MS's "bread and butter" I was referring to business software sales of offline office suite. MS's response to google's entry into that market was to ramp up their own online versions of their more familiar office products.