I see this comparison so often, but it really is apples and oranges. I love gmail and use it daily, but it isn't a replacement for Exchange. First, gmail is just a client... you have no control over the server aspect with Google. That is fine for a lot of small businesses, but for most mid to large companies that isn't an option.
Does Exchange cost a lot of money? Yes, but with that you get a first class messaging platform that you control.