Charge what you feel is right for designing it. Print shops adjust their prices by how many you order. X dollars per hundred or per thousand. X for black, X+ for multi colors.
Concider the ink and wear and tear on your printer if you plan to print it.
If I were you, I'd call a print shop, or stop by with your design and get a price quote from them, then just factor in your time for designing it and the leg work involved.