I would say to add up the cost of the materials you use, labor (maybe $10/hr), and mark-up (maybe consider the difficulty of the piece or jut a flat $ amount). Remember to consider that the person is buying a one-of-a-kind item.

For small, quickly made items such as earrings, I add the price of materials + $10 for me +, if in a show, 1/20 the cost of show entry fee. For large, upscale, intricate, expensive items that take considerable time to craft, I pay myself at rate of $0.75/minute.

A website I found online indicated that using the price of materials as your cost... multiply times 2 for wholesale, times 3 or 4 for retail. This is just a place to start. Depends on your area.