There are free-as-in-beer email servers, even for very high volumes of mail, that any competent IT staff could maintain with minimal effort and better reliability than GMail. How much money do you think GMail would save? Is that amount of money actually worth the hassle of dealing with GMail?
According to a Forester report, they estimate that it costs on average $25.18 per month per user to provide email services in-house, compared to $8.47 for Gmail.
Interestingly, most people couldn't actually guess what the real cost of providing email services in-house was, many guessing $2-11 per user.
The upshot of Forester's analysis was that up to around 15,000 users, it could be substantially cheaper to outsource email as an infrastructure service.
Admittedly, there can be a lot more to the calculations though. Depending on your business needs or industry, you could have regulatory or compliance requirements that might interfere with an outsourced solution if the vendor can't meet those requirements.
The Forester report: http://www.forrester.com/Research/Document/Excerpt/0,7211,46302,00.html
Arstechnica report on the report: http://arstechnica.com/news.ars/post/20090108-report-gmail-about-one-third-as-expensive-as-hosted-e-mail.html