Average Sign Costs. A typical custom steel sign will cost $20 to $30 per square foot of sign. Aluminum and plastic signs are cheaper than steel, typically $10 to $20 per square foot. larger sign will cost much less per foot. Neon signs are great ways to advertise for a business or spice up a storefront. Prices will vary with the size of the sign, how many letters or words and the complexity of any pictures you want on it. Typical costs for a custom 18” x 24” neon sign range from $200 to $400.