You wouldn't use injection molding for a pallet - it's too big. However compression molding is an option. It's not as accurate as injection molding, but it's significantly cheaper. Or at least it is at the scale my father-in-law works at when he makes plastic pallets :). His pallets are custom designed for a specific industry to fit in their manufacturing and logistics workflow.
I thought about this from reading about some of the materials the architect Shigeru Ban works with. He has skinned buildings with board made from hot rolling a mix of plastic and paper from packaging waste.
