The Institute of Fundraising does not recommend the payment of commission to fundraisers.
It is best to agree a contract between a charity and the fundraiser, including details of work to be undertaken and a fixed fee.
More information is available in the Institute's Code of Fundraising Practice Payment of Fundraisers on Commission Basis (145kb).
The Institute also produces Standard Forms of Agreement which can be used as the basis of a contract.
Also provided is a Directory of Fundraising Consultants.