These days there is no such thing as a standard rate. Last time I was looking I was offered such a diverse range of rates I stopped even trying to pin it down. I was offered one interview for a contract that payed Â£15/hour and I would have to live away from home. That would hardly cover my hotel bills
The best thing to do is ask a few different agents to try and get a feel for it. I never specify a rate to agencies. I tell them to pass anything to me and I'll tell them if it is unacceptable.