In this article from Cnet.com there is a little reference to Serena Software (800 users)
Just last week, Serena Software said it was switching to Google from Exchange in a move it said would save it $750,000 a year, according to several reports.
This equates to about $950 PER USER to make this move.
In an article in Eweek the explanation gets deeper, for $50/yr per user the cost for GAPE, the premier service
GAPE includes Gmail, Google's Postini messaging security software and 25GB of mailbox space, as well as greater uptime and 24/7 phone support. It also includes other Google productivity and collaboration applications, such as Google Docs word processing, spreadsheet and presentations, and Google's Sites wiki.
(I love the 25gb mailbox and who wouldn't?)
As noted in the article Serena is really only using Gmail right now because they have Sharepoint installed but are seeing a progression to the online apps. You will notice there is an implied additional expense if and when Serena moves the other 750 people to GAPE's sharing software. 50 are test users for now.
Google is learning to crawl and it's only a matter of time before they stand up and start running....possibly after your clients or organization or after Lotus and Microsoft's core clients.