Having been a small business owner in the applied arts field, I say consider the cost of your materials (including a portion of bottles of paint, glue, etc.) and the time you spend making them (including the shopping and the gas to get there/back). Decide what you think is a reasonable hourly "wage" and then add packing materials (and time spent packaging), because etsy buyers get cranky when you charge more for shipping than the actual shipping costs.
I bet you could see these and especially DotD versions very well on etsy.
That's lovely! Depending on the complexity of the design, I would price them around $15-25 depending. There's a shop in Old Town Albuquerque specializing in Mexican arts and crafts that sells skulls in similar style for Dia de los Muertos in that price range. Smaller, plainer skulls are about $10-12. That you seem to be concentrating more on the Halloween aspect design-wise would absolutely guarantee you interested shoppers, IMO. Especially on these boards.