That depends on the use. For example, 1-2 million triangles/quads on a character is different from that much on the environment. This is because of dynamics and physics. A character will be moving, morphing, shifting (plus any hair, belts, straps, cloth, etc, etc), where as a rock mountain is a stationary prop that can have so much thrown at it (as was the case with this demo).
So, the cost should be minimal.
It's triangles and or quads because those are the best geometric figures to make shapes with right?
Or does it also include that those figures have the best/better aimed reflections?
Dev noob right here. :P