There is a lot of trial and error that is going to go into your attempts to establish a business that may very well end up changing the world or at the very least creating a situation wherein you can obtain a reasonable enough source of income for yourself at this current point in time. Since there is no way to start a business without failing a few times and experimenting until you finally come upon optimal processes, it is important for you to come to terms with this and factor it into your planning process without a shadow of a doubt.
For example, even if you feel extremely confident about the metal business cards design that you have worked so hard on, suffice it to say that when the cards are printed they might not look the same as they did in your head or on your designing software. As a result of the fact that this is the case, you might need to get a few batches printed before you can safely say that you are done with this specific task, so knowing how much each batch would cost is likely going to be something that you would want to jot down in a notepad that you can refer to later.
Business cards usually cost a couple of dozen cents per card, although this cost tends to go down the larger your batches are. Printing larger batches can help you save lots of money overall, but make sure that you start with smaller ones while you are still trying to come up with the best design so that your final batch is worth it.