I charge 15$ for design, shipping should end up around 15 or 20$ for a small sprocket. Manufacturing cost should be around 40$ for a small piece like that.
Here are some sprockets I've made a few month ago for @Greg and Sammy
. Made of stainless steel, so they won't ever rust and the material is really strong. Can be polished to mirror finish, painted or ridden as they are. You can even experiment with heating them up and getting a rainbow finish on them. I can paint or polish them for you too if you want, but for a bit of extra cost