i agree with Mark. this is when it is really important to have your portfolio up to date and full of a good sampling of various products and materials for a large variety of applications.
when you encounter this make sure that we are indeed comparing apples to apples, in my experience most of the time we are not. the client goes to another shop to get a quote for comparison and the other shop prices a different substrate, doesnt include art charges, etc etc.
take a look at the other quote and explain to the client the differences and if the new shop's quote meets their requirements bid the job using similar materials and job specifications.
if the other quote is truly apples to apples as far as specifications go and price is the only issue. it is time to sell yourself and why you are worth the difference in price...we all make custom products and not all things are created equally in this industry.
show them why you and your products are worth more than the 'other guy'...in my opinion you do not want to get caught in the trap of selling
signs for the price of materials and labor. my goal is to sell products and services based on the value of that product to the client...if i am working on a wrap for instance i want to show them why my design meets their advertising goals, how it portraits their company to the public and how much more exposure it will get them versus a television commercial, radio ad, yellow page listing, etc.
it is a different ballgame when you have earned a reputation for selling advertising that generates results versus
signs for the price of materials and labor...people will pay for results.
the goal, in my opinion, is to have your clients come to you saying 'i want to advertise to this group of people' 'i want to increase sales of my xyz product' 'i want to increase traffic through my business on weekdays'... versus clients that come to you saying 'i want a 3X6 banner that says Ladies Night with some graphics on it'
when you prove to your clients that you can generate results for them. when you prove to your clients that you understand their business and their advertising goals...they will pay your price for your services.