You'll have to rebuild the surface by excavating, adding material, and releveling the floor. Besides affecting the horse's limbs, an unlevel floor increases bedding costs. You tend to apply extra forkfuls to fill in dips. While prices vary among dealers, the size of the stall will determine the overall cost of mats. Some owners choose cheaper alternatives.
Some people even install used conveyor belts in horse barns.
You might need only two large rubber mats for a 12 x 12 stall, or a half-dozen 4 x 6 sections. In choosing the size, ask yourself if you'll ever move the mats. Compare this to hefting the 90-pound 4 x 6 mat. Proper installation of mats can make the difference between a trouble-free floor or an irritating and unsafe surface. You want the floor to remain flat and level, with tight seams.