I read the books as a child, too, and I appreciate when those quotes come up. They demonstrate something about the books that never became clear to child-me, something I only dimly started to grasp when I was older and only fully realized a lot later: Those books go far far beyond an entertaining story. They are a pointed critique of society and politics. They have an opinion. They're not mere books for children.

And the point Samuel Vimes is making goes far beyond a simple "If you buy cheap, you buy twice." It extends that basic knowledge by realizing that some people have no other option than to buy cheap, forcing them to spend resources over and over and over again and still get a worse result in the end, and how this is a fundamental and systemic social unfairness.