There pretty simple, and cost about .27 each when I make them.
I use the craft felt for Joann Fabric, and cut 12"X 12" squares, Then I fold over a in at the top and sew along the edge of the fold. The fold them in half and sew the bottom and side. I use throw rings I purchased from Amazon to keep them in place.
I have a sewing machine and a overlock machine which make it really easy.