How Much To Charge To Manage PPC Campaigns?

I recently sold one of my sites to a competitor and now they want me to do some SEO/PPC/Social Networking stuff for them. So I’ve never done this stuff for someone else, so I was wondering how most people figure out what to charge for PPC campaign management? Most of their campaigns are already set up, they just need someone to tend to them and tweak them for maximum conversions.
Should I charge hourly or a set percentage of the ad spend per month?
Thanks for any insight!

Original post by natekapi