what shoul i pay to remove walls to open up a room

Because they're there. Whether old or new construction, you pay for anything to go up or to come down. Although, if it's new construction and they violated the building plan, then and only then should you not pay. It you changed your mind, you're SOLELY the problem and the cost.

- But, to explain further if you made a change (change order). Removing walls is not AT ALL as just simply removing them. Any building is a chain of links relying on each other and was designed to work only that way. Many times drastic measures MUST be taken to bypass any removed links.