Currently trying to find any kind of Open Source collaboration Server - I can assure you that the software costs alone are within 5% on any of the decently known and supported alternatives such as Zimbra, Zafura, eGroupware, open-Xchange etc. Zafura is the closest in terms of quality, but based on my testing, I have noticed that with my 15-user test groups (all users using at least three devices to sync continuously) Zimbra is the closest to Exchange in terms of efficiency, and if you remove OS resource usage I have noticed that the Exchange daemon is the most efficient. I hadn't gone through any kind of upgrade testing to see how easy that is (which could alone still sway me away from my current direction of updating Exchange) but when compared to how much easier it is to tie Exchange into Active Directory and properly apply any domain-controlled policies to clients, Exchange wins hands-down in any system that isn't wholly unix from the ground-up.
In conclusion, you are misinformed if you think any kind of FOSS system can compete with Exchange. If you want any kind of collaboration utilities (Calendar or Contact sync and grouping, etc) then you discard the F part and usually the OS part too - and the supposed knights of FOSS are even more greedy than Moneyholesoft from Redmond. At least Redmond allowed us a 3-month trial with 60 users to test out compared to the others. VMWare let us try Zimbra with 30 users for 30 days before they wanted to charge us - nobody else would even let us trial their packages at all.