Answer 1

There are a lot of factors to look at. Some to consider are:

How long is the script? How long will it take you to record - (and don't forget that the editing part will probably take twice as long again, on average).

Then work out what hourly rate you need and quote accordingly, remembering to take into account your experience in a particular field/in general.

Voices.com has a guide to rates here to give you an idea of what's normal.
http://www.voices.com/voice-over-rates.html

Hope that helps!