As a minimum, perhaps £500 (that's £20 per head for the hour, plus a fiver each travel costs, assuming that it's fairly local) if the choir are actively seeking new items for their repertoire, and new opportunities to perform (so that you don't get charged for rehearsals, etc). In practice, perhaps double that.
However it's really a 'how long is a piece of string' question. We've got an excellent community choir around here, which (based upon when I last saw them) has around 130 singers performing together, usually just on the basis of covering their expenses. Other choirs might expect the sort of payments, per member, suggested by F-F as a bare minimum (with additional money to cover arrangements and rehearsals).
Have you got a particular choir in mind or, if not, can you tell us which part of the country the wedding will be held in?