I agree with you. I think kits are too expensive and going the homebrew route is generally more fun and educational. But to answer: ePaper is expensive, $15-20 for that module. The rest of the components are cheap. You're paying for some profit for the designer, amortised development time, assembly (by hand?) and the fact that it should work straight away (i.e. so you don't spend time). Tindie also take a cut, 10-15%? Shipping isn't free for the seller either.

At work I grit my teeth and suggest we buy the dev kits because it's worth paying $100 for an official board to check something works tomorrow, vs 3 weeks for a PCB and 2 days of tinkering. But at home, as a hobbyist, I mutter under my breath and fire up Eagle.