The cameras are the same, the lenses are different. If there is a $200 difference it could be in the construction of the lens. One may have a plastic mount vs. a metal mount. The cheap Canon lenses can have a plastic mount.
Canon EFS18-135S: 18-135mm lens (IS) filter size 67mm, max. diameter x length 3.0 x 4.0 in.
Canon EFS18-55S: 18-5mm lens (IS II) filter size 58mm, max. diameter x length 2.7 x 3.33 in.
The difference between the two is the focal length, diagonal view, number of lens elements, etc.