Our organization is reaching around a 100 email addresses (60 odd user addresses + assorted ones i.e. [email protected], [email protected]) along with about a dozen email groups.
Currently we are using Google Apps, at $50 per email address per year. Right now, that's $5,000 a year. At our current rate of growth I'm estimating we may need up to another 50 addresses.
We use Atlassian Confluence for our wiki system and Atlassian Crowd for SSO, so Google Apps is offering us little ROI save for the administration burden. (I'll add I am the defacto CTO and am still in process of hiring. We are getting a backend LAMP Developer first and then in six-twelve months a sysadmin and network engineer).
Is it worthwhile at this point to switch to an internal email solution (I.e Zimbra?) which could be hosted in an external datacenter, or is it still cost-effective to use Google apps?
My 2c - unless you're in the business of email somehow, or need the infrastructure potentially to support something else, stick with Google Apps. The administration overhead of moving to your own system without having a capable sysadmin managing it is LARGE. Spam and anti-spam measures have increased the barrier to entry of running a mail server to a large level.
FYI, I say this having worked as a sysadmin and mail server administrator (to the tune of 10's of millions of messages a day)
If email is a critical part of your business and you don't need the side benefits of running your own mailserver, Google Apps is saving you time and therefore money.
My general rule of thumb is: If you're requirements in the near futures (<1y) are unknown and you do not have strong internal administrative staff (a SysAdmin) then outsource the need (Google Mail, Exchange Hosted, etc).
The same applies to the contrapositive: known requirements and strong admin staff, host it internally. When one or the other is there, usually stick with what you've got, but we really don't know your company as well as you do, so it's hard to make the call.
At this point, unless your email system is providing your business some impressive value-add, it's probably best to think of it as a commodity. It's going to be hard to find an ROI that is better than an outsourced solution unless you have a very compelling argument as to why you need to bring it in-house.
With the Google Apps, you have some flexibility should you change your mind down the road. But without dedicated sysadmin support and some specific value-add that you can justify in savings, I don't think you'll beat Google's price once you factor in hosting, hardware, and the opportunity cost of your sysadmin not working on revenue-producing activities because your mail server's IP is suddenly on a blacklist.