There are a couple of things to consider when comparing the two. OS licensing costs and perpetual updates.
First, with Zimbra, you're not necessarily tied to a specific OS. Sure, it needs to be UNIX based, but you've got options. You could go the Apple OSX server route, which gives you more expensive hardware, but allows you an unlimited client license for $999, and I don't believe that you're tied to any license term. You could also go the "enterprise class" Linux route with SUSE or RedHat, but you will have to pay for support for those. If you're being cost conscious, you could download Ubuntu, which is a supported Network Edition OS, and doesn't necessarily require a support contract to run. That would save you more than $5,000 over a 4-year term according to the spreadsheet mentioned.
Second, Zimbra's licensing is more like Microsoft's "Software Assurance" licensing, which allows for upgrades to software when new versions are available. This ups the costs of licensing pretty significantly. Looking that pricing up, it increases the cost of Exchange server itself from $3700 to $5500. That's not the only kicker...the SA term is only for 2 years. That means that if you want 4 years of potential upgrades, it's going to cost you $11,000 just for the server software for 4 years of upgradability. That doesn't include the costs of SA for the server OS, and both sets of client licenses you need (Windows CALs as well as Exchange CALs). I guess you could argue that you don't need the SA because MS generally stretches out their product cycles so that you wouldn't have to upgrade over the course of 4 years, but is that really a benefit? Considering all of the features that Zimbra's implemented in the 2 years that I've been using it, I'd love to sit down in 4 years and compare what Zimbra's got going on to Exchange 2007!
There are many reasons to consider both, but from my personal experience, Zimbra tends to perform better on less hardware for the same amount of users compared to Exchange. Add to the fact that it's based on open standards, and you get a package that can run side by side with several other products, and is very extensible. On our server, we're running an Oracle database, Zimbra, apache, a support ticket system, and an SSL-based VPN. Try doing all of that on the same Windows server!