I have a hot water dispenser for my kitchen sink that needs to be replaced (dispenses 200°F water on demand). The Insinkerator brand offers a water filter “system” for over $120. The system is just a cap with the hose fittings, and you screw the filter cartridge into the cap. Replacement filters are ~$50.
WTF? The filter will extend the life of the tank (I’ve gone through two of them). But it doesn’t seem like it should be so expensive.
Filters for my refrigerator are similarly expensive. It’s just a filter smaller than a can of tennis balls.
Same business model as printers: lock you into an ecosystem and then gut you on refills.
Likewise, glucometers are cheap, diabetes test strips are relatively expensive.
That’s how they make money.
Because of the reference to tennis balls, this thread has a tag of “tennis.”

I think King Gillette invented the approach; sell the razors cheap but charge dearly for the proprietary replacement blades. If possible, switch to a whole-house water filter that uses generic filter cartridges.
yep, for all the reasons stated above. And, many of the designs are patented, preventing competition from low cost generics. see, just by way of example, US8137551B1 - Push filter with floating key lock - Google Patents
which has been the subject of various patent infringement cases against various generic filter manufacturers.
We have a 15 year old Whirlpool refrigerator that insists we replace the water filter every six months…or maybe something terrible will happen? I don’t know. The filters have gone up in price over the years and now cost around $60. I have now spent more on those filters than the cost of the refrigerator itself.
I have a Kenmore fridge that is still chugging along, so I replace the filter whenever the light turns red. (I failed to do this once, and I’ve always blamed that for an ice plug that formed in one of the lines in the freezer.) I have given up on buying Kenmore-branded filters; the last few I bought leaked horribly. I’ve had decent luck with aftermarket filters on Amazon; don’t buy the cheapest thing, and read the reviews.
Can’t you just ignore the filter, particularly if you don’t connect the water supply line?
Well, in OP’s cae, it’s a water boiler, so not connecting the water line is not very helpful.
But a refrigerator filter is not needed if the fridge isn’t connected to a water line (or doesn’t have an icemaker and/or water dispenser to connect). But obviously, some people value having those amenities even if the filters add to the cost, and “just don’t do that” isn’t a very insightful contribution.
And then printers, with printer ink/cartridges.