legal – How much should be paid for an image copyright claim?

We are a small media website based in the UK. If a member of our staff uploads an image without proper copyright and then we receive a request for reimbursement of the costs of a lawyer, acting on behalf of the rights holder, what method can we use to decide the amount of fees to pay?

We can assume that the lawyer will often want to charge as much as possible. We therefore often have to haggle without having a significant scale.