A less expensive approach to this look would be by stacking foam board of various depths wrapped in spider lath (acrylic webbing) and plastered with white portland and quartzite sand a white sand used in plastering.. I'd add a little plasticizer in the mix. You can get color shading by mixing water based paint in the water before mixing in the cement. This is will be a two coat or more process. I use a white thinset for base coats.