You need a good set of scales that weighs light weights accurately and the answer to a couple of questions. Are you going to remelt sprues and runners? Given that, if you remelt you will still have some left overs that will fall to waste. You need your best guess at an overall waste percent. If you are not going to remelt you need the weight of an injected molds total plastic use and the number of baits made. The weight of plastic is real close to water so a fluid ounce of liquid plastic will weigh almost an ounce. Then you can do the math given the cost per gallon ( 128 oz. ) of the plastic. Don't forget freight if applicable.