The sets that are basic ANSI [or sometimes ISO] are more affordable because they are producing 104/108 keys to a "universal" Majestouch-style layout. An operation that is geared up to mass produce ANSI PBT keycaps can churn out all sorts of sets over time, and they don't have to innovate molds.
However, if you are buying a set that includes custom layouts, the cost can go way up. A few non-standard keys are not such a big deal, but to accommodate 65%, 70%, and 1800 layouts means a number of non-standard keys. Fewer manufacturers are geared up to do that, and they have to recoup their investment and production costs.
So, you can buy a nice set of 104/108 Varmilo Cherry-profile keys for $60. But EnjoyPBT 9009, which contains all sorts of non-standard layouts and some extra colours, costs literally twice as much.
The key types are one issue. Then, there's the number of PBT colours involved. Most sets use 1-2. But ePBT 9009, for example, uses 4.
Then, there's the dye-sub quality. This is a major cost factor. Competent dye-sublimation is the difference between dev/tty R1 and dev/tty R2. KPRepublic produces a 9009 set very similar to ePBT. But KPRepublic legends are often runny, blurry or distorted, while ePBT less so.
Then there's the quality of the molds and the production process. Again, KPRepublic is said to be rougher in construction, with modifiers more out-of-shape. PBT modifiers are easy to screw up, even a $120 ePBT set can come with bent spacebars. So you're paying for the production of spacebars until they get it right, and then inspection and QC (and they still screw it up).
So those are the costs that are involved. To learn more, compare a set of KPRepublic and ePBT 9009 some time. Or go look at a cheap PBT version of "GMK Carbon" vs. the real thing.
I have a $20 set (including shipping) of double-shot PBT from Aliexpress on the way. The keycaps are a PBT version of Sky Dolch. I bought it just to see what you can get for $20. Even in the photos, the lettering is not perfect, and the reviews say that the keycaps are rough and 'need smoothing.'