It's like you say, it depends on the surface. Only you can be the judge of this based on how you work and with the equipment you use.
We did a 200m2 area last week in less than 1 hour, 2 whirl-a-way's and swill. It was York Stone but was sprayed with hypo too. Just general soiling.
m2 pricing is not the way to go in my opinion.
I like to think that that's one of the reason's why we have 5 weeks worth of work booked in both commercial and domestic.
We can do a job faster and more efficient than many other pressure washing companies in our area.
They will be pricing either on m2 or like us by the time it takes them, which is longer than us.
We'll be cleaning 2 driveways today. One is costing £150 and the other £250. The first will take an hour the second about 2 hours. That's with 2 men and 2 machines.