Milk jug route
I prefer making silicone molds and then using plaster of paris.
You can make very decent molds with just 100% silicone caulk you buy from hardware stores and some corn starch (look up ProtoPutty on Youtube for a tut), just remember to plug any gaps and holes in the skull before you place the mold else your will never get it off lol.
After that you can make any amount of casts you desire. You can even press paper mache into them to make paper skulls for next to no cost or even latex to make latex skull masks.
EDIT: Forgot to mention Thermoplastic pellets I found out recently are a god send for making molds/props as well. You can get a bag of them for under a tenner on amazon that would easily cover a skull and all you do then is add hot water, mold it on to what ever you want to cast, let it dry and boom almost instant high quality plastic mold and it's reusable after on top! You could also sculpt your own skull with it if really wanted to lol. The stuff really is a prop makers dream come true.