I highly recommend SignCraft magazine
signcraft.com
When you subscribe, they will send you a free gift of their sign pricing guide.
The magazine is worth every penny alone and the pricing guide makes it just that better of a deal.
One thing to note about the pricing guide is that it is on the high side of pricing. This gives you an advantage of showing a customer what it should cost and you can adjust the price according to your town's economic level and competition.