Flat rate for a service. Lulu's idea is a really good one, ring up a few places and see what the 'going rate' is for different services, so you can offer competitive rates. It might not be a bad idea to ring the places you are interested in cleaning and just finding out who does their cleaning and how much they pay (some will freely offer the information if there is a possiblility that you'll provide the same service and much/beat the price), also ask if they are interested in another company. Be careful about random flyer drops, as you might end up targeting companies that are loyal to their cleaners IYKWIM.