standard sizes like A4 and letter paper are in supply and in widespread use, so the economy of them is in a different regime than larger sizes. super back of the envelope bullshit story:> scenario 1: there are 100 million people who want letter sized prints. so they design and set up machines and supply chains to create 1 million printers and paper of that size.> scenario 2: there are 100 thousand people who want 20x30 prints. so they design and set up machines and supply chains to create 1 thousand printers and paper of that size
assuming the machinery and supply chains are the same between the two scenarios then
a customer buying products from scenario 1 is paying one-(100 million)-th of the (machinery, design and supply chain overhead) cost.
....but a customer buying products from scenario 2 has to pay 1-1000th of it. That is 1000 times more overhead-per-customer than scenario 1.
it's amazing you can get it for under 20 dollars.