I also agree with LC 500 plastic. I dont know how new you are to pouring and mold making, but a slug-o is one of the easiest molds to make as well as pour. I dont have acess like you to a machine shop so I use RTV silicone. You can glue down 5 or 6 baits in the bottom of a shallow tuperware type pan, pour over your silicone and your done. With a muti cavity mold like that you can really crank out some baits.
The 9 inch slug baits do consume alot of plastic but you will get alot baits out of a gallon. Save your tore up baits and re-melt them. Usually most slug baits are poured with a white or clear type belly which you can use your virgin plastic for. Add color to your re-melts for the darker top portion of the bait. I use re-melts for dark colors like black, browns, greens, etc. Especially on saltwater or big baits where you dont need super detail, vivid or transparent colors.