Scope makers usually mention a couple of things. First, glass, i.e., distortion from the lenses themselves. As an exaggerated example, remember how a miniscule defect in the mirrors caused major problems for the Hubble telescope, distorting our view of outer space? Same with scopes, and cheaper scopes will tend to have more significant distortion. Luckily, we aren't looking at distant galaxies and most of us will never notice except in the very cheapest scopes.
Then, coatings. More and better coatings apparently make a big difference, although personally I don't see how coatings would add significantly to cost/price. Maybe more coatings means more processes to the manufacturer, hence higher costs?
I'd add a couple more. One, that the quality of the internals, particularly the adjustment turret mechanisms, would also make a big difference, class of fit for the screws, hardness of the materials, and the like. Oh, and etched glass v wire reticle, or god forbid,a painted reticle. Remember when we could choose a cheap, disposable harbor freight tool for a one-time job, v an expensive US made tool that would last forever? Kinda like that with scope internals.
Finally, the tube itself and the bells at each end. It's actually hard to make a tube that is perfectly concentric, inside and out, throughout its length. The more perfect you make the tube, the more it costs.
My $0.02