In doing some research on another topic, I stumbled across a cool blurb on Michael Arrington's TechCrunch about a company called PayScale and a web application they created called "Meeting Miser."
If you've ever been in a meeting, looked around the room at the assembled people, and tried to figure out what the meeting was costing you, Meeting Miser is a utility that will calculate the "hard" costs of a meeting based on the positions of the people involved, the location where they work, and PayScale's enormous database of salary information. You can also manually enter/adjust salary amounts if known.
On the right is an illustration showing the cost of a meeting in Waltham among a VP of Sales, CEO, and COO: $2.82 per minute. Meeting Miser also has a timer that you can start and stop during the meeting (try putting that up on a screen during your presentation!).
Of course, this question typically only arises if the meeting in question is bogged down or not very productive, so perhaps it's more important to focus on improving the quality of your meetings. There are more resources on this topic than I could possibly point to, but I'm a big fan of authors like David Maister, Dale Dauten, and Seth Godin, each of whom has advice for running more effective meetings and presentations. (Seth's ebook and blog article on "Really Bad PowerPoint" are priceless)