It depends on what type of wood you choose. I just helped a client get their home ready for sale and we replaced the carpet with bamboo. It cost them $8 per square foot. Stairs are more expensive. The treads were $40 per lineal foot and the risers were $10 per lineal foot. They did a total of 425 sq feet of flooring which came to approximately $3690 (not including stairs) including removal of existing tile/carpeting, subfloor prep,and some additional items.
We chose bamboo because it was already installed in other rooms in the house. But the good news was that bamboo is less expensive than hardwoods. You could spend $10, $20, maybe more, per square foot, for some hardwoods.
You can also look into reengineered hardwood floors. I believe they are less expensive. The top is hardwood and underneath is plywood. It’s less thick than a real hardwood floor, but you can still sand them down a couple of times and refinish them unlike laminate floors.