I personally don't consider those very challenging - you just need some XPS polystyrene, a Stanley knife and some blades, some sanding paper, glue and some cocktail sticks for extra stability.
Those took some two hours (plus drying time), and given it is such a shitty job, if you invest some more time in sanding and texturing, you can easily produce such things. In contrast to other terrain pieces, it doesn't take much skill, just perseverance.