Below is a how-to for transforming regular styrofoam wigheads into affordable monster, witch, or whatever face you want static prop head using a product called Celuclay, which is a paper mache type material.
Celuclay can be found at any arts and crafts store as well as Walmart.
A 5 pound bag runs about $25.00 or a 1 pound bag for $5
A little goes a long way with this product.
Keep in mind that this will make your wighead very heavy depending on how much Celuclay is applied to the form.
For lighter weight wigheads you may want to try Model Magic also available at the above locations.