A heated discussion is raging in the comments to a Clint Boulton eWeek article about Serena Software’s decision to switch from Microsoft Exchange to Google Apps. What is at issue is the true cost of running Exchange. Serena says that junking Exchange in favor of Google Apps will slash its costs from $1 million a year down to $250,000. Many of the commenters are wondering how an 800-employee company manages to blow an eight-figure sum on running its email servers, and the debate throws an intriguing light on how people evaluate the relative costs of on-premise software against in-the-cloud alternatives. Here’s one of the commenter’s calculations: