You can buy rubber molds for stepping stones for a buck or two. The imporant thing is what you put into the mold, how you consolidate it, and how you cure it.
A manufacturer pays about $20,000 for a single mold in addition to the $2,000,000 to $5,000,000 for equipment to use the mold. They are lucky to get 250,000 units out of it before rebuilding.
Using your own molds, paying for aggregate deliveries or running to HD for bags to make concrete things is very educational, but you will not save any money. - If you want to make something unique, that is a totally different thing.