hiya
I would price it per stairwell.
if you do it like this then the customer only considers the smallest amount, if you said 10 per stairwell the customer will think 10 thats not bad, but if you price the hourly rate times the amount of hours he sees a much larger costing and he might think 50 sounds a lot.
even though he will pay the same either way it helps to break it down so he sees the value side.