Mostly they will give you a quote based on how long you want them to perform for and how far they will need to travel. Very approximately think around £200 for a three hour set with breaks plus travel and expenses.
But ring and ask, they won't mind giving you a rough idea. I think they negotiate regarding the money in the hat too. But some don't like to have a hat while others use them all the time for extra cash in hand ( don't have to share it with their agent)