How to Manage More than one Ad words campaign

As a company we provide SEO services to a great number of clients. However as yahoo and msn are both bringing a new form of ad marketing program in a similar line to Google's we are looking to start providing PPC management as a service.

I would love some information from anyone else that manages multiple ad words campaigns either for themselves or better yet for multiple clients. My key questions / thoughts are:

• How easy / time consuming is it to manage say 10 campaigns at once?
• What kind of pricing structure do you employ? A percentage scale or a fixed monthly fee?
• Essentially any information on managing and charging for managing multiple campaigns.

I really look forward to any information people can supply. Either links to other stories / articles etc, or straight from the head thought.

Many thanks in advance.

TTFN
Dave