I get a lot of questions about narration for e-learning programs, particularly adding narration to PowerPoint. Here are a couple of sources for voice talent that will give you an idea of the voice talent available (both North American and British English) as well as ballpark parameters for pricing. Do-it-yourselfers might find some inspiration and guidelines here as well.
http://www.voices.com/
http://voice123.com/s/voiceovers.html