There's no point in making him another sign at $40 if it's not worth your time.
This is not a charity. But you need to keep in mind that one large reason for the repeat business is because you were so inexpensive the first time.
I think you are going to have to raise your prices, and accept that you are going to lose some customers. Good luck. It's difficult to start out with low prices and get the consumer to accept paying more. He's going to counter with "hey, the first one only cost $40, you're screwing me".