Opening up, like removing walls? That requires a lot more info. Is it load bearing? Are there pipes, hvac, plumbing or anything in the wall?
Optimistically, if the wall you want to remove to “open up” the stairwell doesn’t have any issues – that part is cheap. The finishes of the other things in a stairwell will determine price – if you have a non-fancy, wood railing – no custom milling on the knoll posts, etc… It’s one thing. If you get artisan crafted wrought iron and fancy milled railings…. that’s another thing.
