You have a problem if, in the customer's mind, you are just one of many signshops - one of many places where they can buy basically the same thing: A sign is a sign...is a sign, right? In that scenario, price will inevitably play a disproportionately larger role in the buying decision.
If you don't create real value for customers in a way that your competitors can't, or won't, then I suggest you start thinking about how you can. What do your customers value? What do they want more than anything else?
I know it's a cliche, but it's a suitable one: Nobody ever bought a drill because they wanted a drill. What did they want? Holes. Nobody wants a sign either. Signs are the drill. How can you help them get what they really want?
You could start by knowing more about marketing than your competitors, and more about designing to help them get more of what they want. You could learn more about each customer's business than your competition - more about their business objectives and challenges, more about who their target market is, and what persuades them buy.
Here's a book that can help you with your problem, and with your customers' problems:
Positioning: The Battle for Your Mind.
I know. The cover is butt ugly. Proof that you should never judge a book by its cover. Here's a pretty good
summary.