I have made his purchase decision one based on quality, not price. He also realizes that cutting corners undermines the durability, appearance and effectiveness of the signage. If you trust his business sense, he'll trust yours. Banners take more abuse than almost any other kind of signage. In addition, banners are often hung by a client's hourly help, most of whom know nothing about proper installation methods.
Banners also receive little or no cleaning.
Just like a car, they need washing and waxing. Automotive paints, by the way, are usually superior to most sign paints in durability - yet car manufacturers will not honor a warranty on the paint if the owner has not provided reasonable and consistent washing and polishing. This would be considered abuse and neglect of the paint. Therefore, their construction and decoration needs to be with the best materials, not the cheapest. Obviously, if a banner will be used for a one-time affair, we can make material choices that will save the client some money.