Glass making is a relatively new tech and as such it isn't that widespread yet. Glass is expensive because of the skill required to handle it, not so much from the materials used to make it. Potash mixed with sand lowers the temperature needed to melt the sand into glass. This allows the making of glass without having access to a furnace that can burn at the melting point of sand, which would also allow the melting of iron. Such furnaces simply don't exist in this age.