The short answer is "
mu".
The long answer is that you can really only guesstimate how many pages it will take. There are many variables which will impact on how much paper will be consumed:
- Paper size (8.5"x11" vs A4)
- Font used, and size thereof ("Arial" vs. "Helvetica")
- Margin settings
- Optional headers and footers
- Browser being used
- Operating system and settings, such as anti-aliasing
- Printer peculiarities (laser vs. inkjet)
So if you can figure out an average, you get an idea. Otherwise, you might want to just suggest using the "Print Preview" function, which is always accurate.
Even if you know the width of the characters, you will have to calculate how images are inlined, words wrapped, and before you know it, you will be writing a Web browser simulator, which is probably an exercise in futility.