My hunch, FWIW, is that pros do not typically charge by the slide. They
charge by the project, by the day, or by the hour. A slide can have one
image and a handful of words
http://presentationzen.blogs.com/presentationzen/2005/09/whats_good_powe.html
or it can have zillions of things
http://presentationzen.blogs.com/presentationzen/2005/11/it_was_one_of_t.html
Of course, there's also the consideration of how quickly one co-worker
performs the task versus another because of skill level, etc. Faster
people can and should cost more.
If you must have a "plug number" for a spreadsheet, I would recommend
calling your local temp agencies like Aquent or The Creative Group for
PPT specialist rates. Internet-based freelance jobsites also offer some
clues. Search on "freelance design" + "PowerPoint."
HTH,
Tony Ramos
Specialist in PowerPoint Presentation Design
http://tonyramos.com
Home of "Tony's PowerPoint Weblog"