We have approx 250 users. GFI MailEssentials and MailSecurity is what I use. The software was approx. $4000. Yearly maint is approx $700. I have it on a Proliant DL360G4p with dual-dualcore and 2GB ram. It's running very well. We have approx 600,000 messages per month.

.. To give an idea of sizing and cost.