The module kits also include foam roadbed, risers, and landscaping materials like groundfoam, plaster cloth, and trees.
I have not tried these products, but they certainly look interesting!
EDIT - Just looked up the price at www.walthers.com - US$125 for straights (18"x36") and US$150 for corners (36"x36"). Maybe pricey, but check the supply list that comes with them before you decide - there is a lot of stuff you might be getting anyway...