Alex,
I sure would like to save $40. a sheet if possible. Does the PVC your using have a slick surface or is it a little textured. I'm not opposed to what your doing. If you're happy so am I.
On the topic of warranty:
Every customer, I can remember, ask about life expectancy and maintenance on their new custom made sign. I'm proud of your business and wouldn't suggest any changes with what you're doing with regard to this topic. We live in two different business worlds. Our customers aren't as price conscious as yours They come to us, in part, for image quality and look more than price.
When a signs is installed I keep a close eye on it. Since they require lots of time and attention and the prices are on the upper end. When a visiting new customer lets me know price is the bottom line, I help them find another vendor. That way everyone is happy.
I'm proud of every sign shop servicing their community. We are all different but equal. My hats off to sign businesses knowing how difficult it is to survive. To make it in this business you've gotta be book keeper, sales person, and designer. Working together is a benefit. Sharing materials and techniques make life more fun.
Joe