Our organization is reaching around a 100 email addresses (60 odd user addresses + assorted ones i.e. firstname.lastname@example.org, email@example.com) 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?