WTD: Stair climbing robot

You'll need to be more specific. Most treaded robots can climb anything, including stairs. Some cleverly designed wheeled variations can also climb stairs. A more complicated, yet elegant solution are legs. And in this case you're right, in that practically no one sells such a robot due to its disproportionate high cost to low usefulness ratio. What's more, few hobby-oriented robotic products do anything useful or complicated. So if a company did sell a robot capable of climbing stairs, it would probably break the bank.

In other words, ramps are still the easiest way to traverse stairs ;)

Size? Price range?

You better have A LOT of money for a robot that can reliabily climb different kinds of stairs. This is still something of a Holy Grail for robotics, and the robots that offer this are usually prototypes that are not for sale.

With enough money it's possible to build a stair-clibming robot. Whether it will climb all stairs, especially while not destroying the surface (carpet, tile, whatever) of the stairs, is another matter.

-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza