With designs like this you can build them for pretty cheap since there’s just not a lot of components, the most expensive item on this build is the controllers due to it being a wireless build and these wireless controllers being in the region of £20 a piece (and it needs 2). But you can do it wired for less. Also I have socketed these so they can be removed and reused easily if I change to another keyboard with the same controller footprint, so that’s not a cost I need to bear for every build, just move the controller around. There are designs that use cheaper controllers too, so yeah, it can be expensive but one of the things I love about these small split keyboards is they’re dirt cheap by comparison. PCBs are like a tenner for 5 units direct from a PCB fabricator in China, etc. So you can have lots of fun without significant cost per item (I’ve still spent a grand or two probably (maybe more, I not counting)), but have a collection of around 30 keyboards to show for it. Most of them are like this, fewer than 40 switches and no case. Great little boards and a joy to use (for me, YMMV).

You’re right though, the hobby sucks you in, much like coffee, or pens, or headphones, or k-pop fandom, or cars, or bikes… etc. you name it, hobbies get expensive if you engage with them a lot (though magnitude varies, see cars). But at the end of the day, it’s an leisure expense, no more of a money pit than any other regular leisure activity really, you’re spending the money to have a good time, if your budget allows, it’s definitely rewarding.