GMK is like a Porsche
Enjoy PBT is like a Subaru, engine blows
D/SA are like American Muscle Cars, ...
A decent PBT-set could be like a Nissan GT-R
While I probably couldn't nail the analogy well, you get the idea
GMK is a safe bet if you can afford it, I would prefer a decent PBT-set, PBT feels better, but each PBT set I've checked has issues, some doesn't have extras, some have weird scoop/bump combos, some are dye-subbed by a baboon, some are dye-subbed by a pro (IMSTO) but has an inferior PBT keycaps and no extras or lack even the commonly used keycaps
My ideal keycap would be Vortex/Cherry profile PBT, without the surface imperfections of Vortex PBT's, dye-subbed by IMSTO, with extras for various layouts
Sounds impossible for PBT, but for ABS, GMK has it all
And to address your question, not everything has to be as cheap as the material, probably GMK sets are optimally priced, at least most are, a bit more expensive and most wouldn't be able to buy them, a bit cheaper and the profits would go down, tho, after buying Carbon for $99 at base, I did develop a hatred against too high markups too
(on to ranting)
I wish Uniqey would sell GMK's directly at one point, mainly for the replacement keycap aspect, you pay $150, but there is a high chance you are going to receive keycaps with various scratches on them, I used to buy 2+ of a set to eliminate this probability, but with strong USD and personal finances, can't do that any more. Can't buy them with trays either, since customs wouldn't like a large package
Well, I started this post to defend GMK pricing, but you made me realise I should probably stop buying GMK keycaps