xkcd

If you don't have a sensor then that is cheaper in the short term, but it quickly wastes a lot of energy running when the humidity is under control. For a dehumidifier you need more energy to get lower so the room well generally will be fairly consistent not too low humidity even as outside humidity changes, but you will use a lot more energy than a system that turns off when humidity is good.

there are purely mechanical systems that used to be used. However they have some weird machining requirements so it isn't clear they are cheaper than a digital system (this partially depends on volume - make more and the machining gets cheaper per unit). The digital system is as we already have established is very cheap and lets you put buttons and LEDs on the unit for a few cents more - this is far more valuable to marketing than the possible savings (if any!) from a mechanical control.