Plus backup, server licence, admins, storage.... Outlook licence. And to add insulte to injury, the licence is even more expensive than direct competitor like IBM lotus note and Novell groupwise.... and that's not considering open source alternative.
Exchange cost a lot.
Backup, admins, storage are going to be required no matter what you're running--even if you're only running postfix and courier. I'll grant the cost of the server license, but that too is fairly cheap (around $700 last I looked). Amortized it across a user base of any reasonable size, and it's at most a couple of bucks a user per year. If it's more than this, your userbase is so small that you should probably be looking at a hosted solution, anyway.
In either case, "horribly expensive" is a gross overstatement at best.