we've been the oposite
In terms of maintenance, I've spent a lot more time and money keeping the Win servers going than I have the *nix servers. Once setup, the *nix servers are mostly just an easy update process and monitoring review.
In terms of initial purchase, I can build six *nix servers for the price of one Exchange dedicated server. A stock Win server using only it's included roles plus the price of CALS probably equals three or four *nix servers.
We'll see how it pans out in the long run but over the last three years, the greater drain has been maintaining our Windows addiction.