I think price is something one can't really set to specific level like with records.
In most places, old porn is garbage. They just want to get rid of it, to get new stuff.
Like said, in Finnish shop there was HUGE shelves full of 1 euro still sealed paperbacks of everything else but incest, beast and kp. And despite the price being throw-away level, shelves always remained full.
There can be international demand of collectors who need the get things, and who won't refuse 25-100usd range for product they feel they can't live without. But I'm assuming that is small market.
While some cities and some shops have problem moving this stuff, as opposition, I recall visiting one place in Paris who sold beast paperback xerox bootlegs for 34 euro each! Its admirable to think someone actually copied 150 pages, stapled them together into bad looking pile of paper and for this work obviously needs decent payment.
I'm not sure was it this site or some other xxx paperback auction site what sold kp topic paperbacks (amongst other) for 100usd each. Then same year visiting NYC, I saw a pile of 3 book bundles of exact same titles, still new stock from 70's, 7,99usd per 3 books wrapped in plastic. To guarantee getting rid of the stuff, they had wrapped copies in a way that if you wanted different titles, you still got several duplicate copies because all bundles had different selections, but always one different and one same as other bundles.
So what is worth of money and what isn't? I think all sorts of successful auctions and sites with big prices will give illusion of some price range what might not be there. You think you got valuable stuff, but it's valuable ONLY if you find the one obsessive fanatic who's willing to accept the deal. Otherwise it's just waste pulp. Which is probably more common case.
I'm not sure was it "porn gold" or other book talking about paperback business and there was said the painted/drawn covers were much much more better selling than photocovers. But I guess due cheapness, majority of paperbacks turned into photocovers sometime in 80's.
If you want good looking paperbacks, 60's, 70's, early 80's maybe. Some specific publishers, especially s/m can have magnificent artwork and design.