You can make your own prosthetic skin for monster makeup really easily! You'll need unflavored gelatin--find this in the grocery store near the jello--glycerin, which you can get at the drug store, water, a microwave and a china or glass plate.
Put your glass or china plate in a 250 degree oven to warm up. Mix two tablespoons each of gelatin powder, glycerin and water in a small microwave-safe container. I used a tea cup. Heat for ten seconds at a time until the mixture boils.
Take your warm plate out of the oven and wipe it with a little bit of vegetable oil. Pour the hot glycerin mixture--VERY CAREFULLY: it will be very hot and sticky--onto the plate and tilt and tip the plate to let the glycerin mixture coat the plate. You want it as thin as possible, which is why you heat the plate, to keep the mixture runny. Cool quickly over ice (don't let the skin get wet) or just in the fridge. When it's completely cool, peel it off the plate and store between sheets of aluminum foil or wax paper in the refrigerator.
Apply to skin using Elmer's glue, and make sure to decorate with lots of fake blood...made with corn syrup, red food coloring and a bit of instant coffee powder for that clotted look.