BigEdMustaphaAs keycaps are the interfacing surface, their quality and cost for a custom set is equivalent to a generic mechanical keyboard itself.
Either way, welcome to the world of mechanical keyboards! If you ever see a drop for GMK Cherry profile keycaps or SP's SA profile keycaps, prepare for sticker shock! They are not expensive without reason, just as everything else in the world, you get what you pay for. Feel free to stick to your other cheaper keycap sets, and the rest of the enthusiasts will binge on completely custom colorways to suit their needs. Your personal take on how much keycaps should cost isn't relevant here, unfortunately. As a capitalist society, if you're not interested, you're free to move along.
CyphreNot to mention, these are high-grade enthusiast levels products in a super niche market. You don't spend who knows how much on tooling and then sell it at a loss because "lol just plastic."
It's just like how people spend a lot on fountain pens and inks, audiophile gear, and up to the thousands for USED cars they own. "But cars can drive me around and makes things so convenient." Keycaps make my fingers achieve nirvana, deal with it.
To the average person, you're being made fun of as well for even spending a cent over $20 on a keyboard. The market isn't for everyone. Just like the enthusiast market may not be for people who are interested in JUST a mechanical keyboard.
Into the trash it goes