schedule C inventory

I have a small business & I am trying to figure out how to do the inventory for my schedule C. As I understand it, I'm supposed to figure the cost of the inventory I have at the end of the year. I had quite a lot of inventory at the end of the year, but I don't know how to figure the cost. I make small figurines and other knickknacks out of bins of beads, hardware items and other things I find in stores (and sometimes on the street). Each piece is unique, each one costs me a different amount & frankly, I don't know how much each costs. "Cost," as I understand it, is how much it costs me to make them. I assign them prices based on my labor & what I can recall of the costs. I know, not very scientific, but what can I do. I'm afraid I'm just going to have to guess the cost of the goods sold... perhaps a percentage. I'm just wondering if there is an easier way. Is there? I don't know why I can't just take my earnings & subtract from them what I spent this year (the first year I've begun keeping careful records) & call that my profit & just claim that! Mae