Again martin is correct, cost in your travel. But you need to cost in a bit more than that. wear n tear on tyres, Van. Travel time.
15 miles is not a great distance. We have contracts now alot further away, 40 miles plus. So easily viable. Just make sure you can cover and have staff that can cover incase you are ill, we generally have sites now with 2 - 3 cleaners on. So if one does not turn up the others can cover.
Dave