How much to charge for PPC freelance service?

Average charges for PPC freelancer service, contract model, etc

[niebla]

I'm a successfull PPC "expert" who has been increasing PPC sales in my company for the last two years (at a 200% rate every year!)

Now I have the chance to have my first client as a freelancer. I know everything I need about how to manage an account but have no idea on how much to charge to my British costumer, who sales international education programs (I'm in Spain). I know agencies usually charge a % of the customers' expense in PPC, but I would rather charge a fix amount. They do not do any PPC so far, so I'll have to implement the whole thing from scratch.

Those of you familiar with the legal implications and "threads" of beeing a PPC freelancer, any "must-include" clauses in a contract?

Thanks so much in advance.