Whether new or pre-loved, do you have a spending limit for a given aperture and optical arrangement?
This I feel is the main determinant on what generally sells refractors of different sizes, makes, and choice of doublets, triplets or quads. Preferences for imaging are different I appreciate, but cost is still a factor. Likewise doubletitis and achromania.
I doubt many of us (or maybe any of us) have rules like,
* Up to 1,000 USD per inch of aperture is the maximum for apochromatic triplets
* Up to 800 USD per inch of aperture is the maximum for apochromatic doublets
* Up to 150 USD per inch of aperture is the maximum for achromats
It's just not that simple. I would pay more for an Astro-Physics refractor than I would for an equivalently sized Explore Scientific or Skywatcher even though they may be identical on a spec sheet. I would pay more for a scope with a Feathertouch focuser than for one with a house brand single speed. I would pay more for a new telescope than a used one. I would pay more for a telescope with an available matched field flattener since I do a lot of astrophotography. I would pay a premium for "compactness" and "low weight" (until I felt the compactness was affecting mechanical quality or suitability for a given use). I would pay a premium for speed--faster scopes are generally a better match to my particular uses.
There are some "features" I wouldn't pay a premium for, though. I wouldn't pay extra for a triplet over a doublet. Either might be better or worse suited for a given purpose. I wouldn't pay a premium for Fluorite over FPL-53 assuming appropriate mating glass for either. I wouldn't pay a premium for air spaced over oiled or vice versa. I wouldn't pay any significant amount of money for a refractor over about 140mm in aperture--it wouldn't get used frequently enough. Made that mistake in the past.
End result? No specific "rules". There are features I care about and features I don't. Many of those features are specific to me and wouldn't extend to every consumer. In order to determine what I would pay for a given telescope, I would need to evaluate how it fits in with the rest of my telescopes, whether it would be used a lot or a little, what additional equipment would be required to make it useful to me (flatteners, cameras, focuser upgrades, mounts, etc.), whether it would be a replacement for an existing telescope, what my then current financial situation looked like, and whether the opportunity is one I'm likely to run into again or not.