No point going overboard with paying them. £5-£10 a customer is a fair price, especialy if they are half decent and can gain 3/4 an hour. Where else will they earn that much for such little work involved??.
Dont start thinking of paying an hourly rate as never works with canvassing, get whoever does it to treat it as price work which will make them try much harder than if on anhourly rate.
Hope to see some good results posted from you soon.