How much to charge to manage ppc campaign?

We have been doing seo and web design for many years. We have done a little bit of ppc for others and a lot for ourselves.
We have a client that wants us to manage decent size ppc program. They spend $10,000 to 20,000 per month.
Any ideas on what we should charge?
I’ve talked to people that charge a percentage and some that just charge a monthly fee.
Any thoughts or advise will be helpful.
Thanks.

Original post by seomagician