Materials + Labor + Profit = Cost
Problem is sometimes Cost doesn’t equal Value. You need to charge what the majority will pay. If you think a design is truly a one-off, it can cost more. If you’ll design it once and sell a thousand of them (with minor customization), then it can be cheaper. For example, I made an acrylic heart sign for my Wife. Some folks at work saw it and I immediately started getting orders for it. ~$15 in material costs with the LED base. I can’t calculate my labor because I suck at Illustrator so it took me way longer to design it than it would if I knew what I was doing. But, in this case, all I have to do to make more is change the names and date and press the button. I’ve been selling them at $30 and everybody’s happy. On the other hand I made something else (that I won’t reference because even though I don’t think it breaches ©, it’s a completely original piece of art inspired by a popular franchise) that cost me ~$30 in materials. The first one I sold the person wanted to pay $40. I told him that at $40 I literally wouldn’t make any money on it… $50. Sold. The 2nd sale of it I confidently said $50 and the guy didn’t blink an eye. He just said “Oh yeah. I want it.”
I suggest price things a little high. You can always come down if you absolutely need to. Much more difficult to raise a price. You just need to justify your price so that it approximately equals the value to the customer.