My cost was actually $0. I saw lots of expensive options online then decided to build one from scrap laying around. It is not pretty but is a good start. I used an approximately 16x16 inch piece of 1/2 inch plywood, a piece of 2x4 and five screws.
You could walk into a hardware store and find precut pieces of plywood on the shelf. Something in the 1x1 foot to 2x2 foot range would be good. 1.5x1.5 feet (45x45cm) is likely optimal. I used 1/2" (12.5mm) plywood for my 63 pound (29 kilo) son. For larger boys I would use thicker plywood. Then you need one or more boards to go under the elevated end. I used a 2x4 (1.5" elevation) to start with. Most hardware stores will cut lumber to length for free. They might even be willing to fasten the boards together. If not screws, nails or even wood glue would work.
As my son loosens up I plan to add additional height. Currently the slant is a little over a 5° angle.
I do plan to sand the edges. I was rushing to build this while my son took his bath.
My son is using it twice a day for more than 2 minutes at a time while he brushes his teeth and flosses.