I've known companies to do $45.00 per staircase.
I think charging per stair is the way to go no matter whay you charge for each. Not all houses have the same amount of stairs.
Same reason for doing square footage price.
When I give them the price, I also tell them why.
"We have to detail the stairs. We get top, front, nose, edges and we are on our knees when we do it. They take more time and effort than other areas."
I have never had anyone give me grief about my stair price.
Now square footage price? that's another story...plenty of price shoppers out there.