Costs? Cardboard for "free" and 3 hot-glue sticks per paint rack. So if you have a bit of time and some raw materials you can do these.
I´m posting some WIP pictures and the final result.
EDITED: I forgot to post a picture showing the rear view. Please note that there are five vertical supports, that way the racks are strong. Another thing is that the cardboard corrugation should be vertical in the supports – that will make a sturdier structure. I think three supports would work well but I have preferred to use five because it holds 100 paint pots and that’s a considerable weight.