I'm by the sf. When charging 55cents per/ the job can easily get up to $300 for 3bdmrm, hall and stairs. Telling ANY potential client that over the phone immediately makes them a price shopper, unless they know about you and/or they have tried other cleaners and realized they get what they pay for.
I've been "stung" a couple times, where I estimated 3 rooms over the phone is one example. Told me 3 rooms, one large, 2 small, figured about 200sf or so, about $120, then get there and the one room is almost 300sf, almost doubled the price. Then what? This is why, esp when they tell me they have a house 2000sf or bigger, its well worth the time, even if I don't get the job, to go and talk with them and measure it up. I may not get the job THIS time, but in the future...who knows. New owner calls and I've got the measurements in my laptop so I can tell them, yes, I know the house, here's what it will be. In a small town, tend to do the same places.
I like to do a in- house- quote so there are no surprises for the client, OR me!