Exchange is $1000 + $350 per 5 additional users. You'll pay that and then some the first time you need to call in a consultant for half a day to figure out why your Linux shareware app isn't receiving e-mail.
With Exchange 2003 you also get an amazing Outlook Web Access. Free Anti-Spam that works very very well. Calendaring, public folders, tasks, easy backup and restore, seemless integration with Windows Networking and Active Directory.
And e-mail is NOT one of those things you want to have to be constantly fixing. Seriously, if Exchange is too costly an investment then how are they gonna be able to afford your salary next month?