Ask HN: How much to pay recruiters if no payment terms have been discussed?

A recruiter sent me a resume and asked if I wanted the person's contact info. The email header said that any communication with the candidate needed to be initiated through them. We decided to interview him and let recruiter know. We liked the person and made an informal offer. How should we handle this? I don't think we have any legal obligation to pay anything since I never agreed to anything, but is there an ethical obligation and how should the fee be arrived at?