Many people think that 3D printing is more expensive than injection molding, but that is actually false. 3D printing materials costs break even with injection molding materials at about a quarter of a million parts, but whether it is cheaper as a whole than injection molding is really dependent upon your specific project.
However, something that is very often overlooked by designers when they create molds is that once you have all those parts made, you then have to store them and ship them. Both of those are no small costs. If you are shipping, for instance, within the continental us, shipping is quite expensive and you'll be spending several thousand dollars to move a truckload of your parts to your warehouse. Once those parts are at your warehouse, it's very expensive to store them because you have to pay rent for that warehouse. These hidden costs are to be considered when you do injection molding. While it may have an upfront cost and then low per part cost, you have long term carrying and transportation costs that go along with it.
Now 3D printing can produce the same volume of parts as injection molding. At Slant 3D, we produce thousands of pieces for different clients, more cheaply than injection molding. The reason for that is because we don't have the mold cost up front, so a client is able to just email us a design and then we can produce up to hundreds of thousands parts. The per part cost is higher than injection molding when operating at lower volumes, but if you're making a quarter million small parts, 3D printing can make them for the same per part cost as injection molding.
On the extreme end, you can even just email the product over, get verification samples, and then just create a listing on your site. When the order comes in, we print and ship that item for you so you completely eliminate your shipping cost and warehouse cost. All of this of course also maintains the benefit of eliminating the upfront mold cost. This on-demand style of part production cannot be matched by injection molding. If you buy that mold, you have to use it, and you have to make and sell tons of pieces to make the mold pay for itself.
So, is injection molding or 3D printing the better option? It depends on the context of your business. If you're going to sell a million parts tomorrow, you probably want to go with injection molding. If you have a more distributed fulfillment schedule, 3D printing is probably a better option because you eliminate all your ancillary costs. Just make sure that when you're doing the cost comparison that you consider all of the costs, such as the mold cost, carrying cost, warehousing, and shipping. A product is not done and not made until the customer has it in their hands. If your manufacturing method can make that process cheaper, that might be the right choice.