As long as you waterproof the corner (depending on your wall material), bout the cheapest waterproof material suitible for a shower would be FRP. It's the bumpy textured plastic wall panels you often see used in public restrooms and kitchen prep areas. Runs about $30 or so for a 4 x 8 sheet. More on doityourself.com

There is usually a version sold at big box stores. You can akso check tile stores. I have done 3 showers now with Onyx. The best advice for installation is to make templates. I rip down 1-1/2" wide strips of 1/4" plywood, and make some 6" squares of the same plywood. Cut down the strips and scribe them as necessary. I nail the strips together with plywood gussets, CA glue, and my 23 ga pin nailer, so that they form the exact layout of the wall. I use the 6" squares with proper sized holes cut in them to locate things like tub spouts, shower valves and shower arms. Once you have the templates built, pull them off the wall and lay them on top of the panels to make your cuts. The panels can be pretty pricey, and the templates basically guarantee mistake free cuts as well as a quality install. More on reddit.com

The plastic ones are awful, the squash when you screw things too them. The more expensive ones are either 12mm ply or mdf covered on the front and back. A far superior product and that’s whey they’re double the price. More on reddit.com

I used a product made by a company called Transolid. (Imo, stay away from acrylic) http://transolid.com/transolid-shower-walls I purchased the shower base from them as well. Shower door came from Lowes. I would definitely recommend the product. It's a composite material and little difficult to work with but the shower turned out unbelievable. I used a jig saw and a belt sander to get the panels to fit perfectly. You can get full wall height size panels (92 inches in my casel) and I used PVC tounge and groove panels for the ceiling. More on reddit.com