Do you know what you have on hand in supplies? Over the holidays I made and gave away a LOT of jewelry and used up a lot of my sheet and wire. I don't keep super close track of what I have on hand and what I may need to replace. I also don't keep close track of costs since I am not in business. However, I do loosely keep track so I don't run out of wire or sheet I use most often. Nobody wants to be caught in the middle of a project, run out of something, and then wait for the order to arrive at your door. By then, your mojo may be gone and you are on to another project.
In the past I used to (OK, I still do) get sidetracked with having fun creating and forget to subtract what I used from what I started with. Now, once a month or so I update my inventory sheets to let me know where I may be running low, erasing the old numbers, changing to what is left. That way, when sales happen, I know what I should order. Inventory sheets, kept in plastic sleeves and stored with my sheet and wire, also give me an idea of what I use most of and most often.