I've got Trend Micro Client Messaging Suite running on my Windows 2003 SBS w/ Exchange 2003 SP2. We've only got 3 users, with 1 user getting the vast majority of the emails. The spam varies widely from day to day. Some days 20 spam emails, other days 100 or so. In the Trend Micro administrative panel, it shows that it's detecting roughly 25% of all emails as spam. But it's obviously missing a lot too.
What should my expectations be for spam? How much should I expect to spend for a spam filter for just one user, and how much of a reduction should I expect?