I can get a board from AliExpress with an ESP32-C3 on it with free shipping for £1.10, so I’m not inclined to believe the £0.765 unit cost for a 5000-part reel from Mouser is really the cheapest way to get them in bulk as the other parts on the same board and the shipping have to cost something.
The cheapest hygrometer that Mouser sell is £0.748 per unit for a 10,000-part reel, and its datasheet says not to leave it for more than 60 hours in greater than 80% relative humidity (which is a pretty likely scenario for a dehumidifier) as it’ll drift, and if it happens often, it’ll age faster. You need to spend more to get rid of that restriction. I’ll concede that the accuracy penalty if you cheap out isn’t as bad as I thought - I’d not actually looked at a datasheet to see how badly modern hygrometers would drift, I just knew that they did - so plenty of manufacturers wouldn’t care, but the parts are still comparable prices, not a factor of ten like you’re claiming.
Don’t give JK Rowling ideas.