To use a friend's book as an example:
Normal People (https://www.amazon.com/Normal-People-Novel-Sally-Rooney/dp/1...)
Paperback: 304 pages, 0.6 inches, 8.3 ounces
Hardcover: 288 pages, 1.0 inches, 14.7 ounces
The hardcover has fewer pages but is 70% thicker and 80% heavier. The other dimensions differ only slighty (5.7 x 8.6 vs 5.2 x 8)
I checked two other recent releases and this seemed somewhat consistent. And even if the weight difference is mostly the cover, I would tend to think the cost of material to correlate linearly with weight (as your accusation also implies). So you are getting about twice the weight for about twice the price?
Printing costs are obviously only a fraction of the price. I wouldn't even be surprised if books were one of those goods where gross margins actually increase even when the price difference is lower than the increase in weight (the canonical examples are perfume and software).
Personally, I don't care too much about paper thickness. The far more relevant quality issue used to be acid, with paper degrading within a decade. Today's paper is good for a few hundred years, so it wins any competition with those hand-sewn, thick, artisanal "quality products" of previous eras.
 Incidentally, the hardcover I looked up above sells for $9.95 while the paperback is $17.00 (both new). Go figure!
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