Every storage instruction on this site ends in the word dry, and dry is the only one of those conditions a person cannot assess by standing in the room. Cool you can feel. Dark you can see. Damp you notice at about the point the flour has already caked.
The number to work to comes from the EPA, which puts indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. That is a mould-prevention figure for buildings rather than a food standard, and there does not appear to be an authority publishing a relative humidity threshold specifically for a cupboard of dry goods. It is the right order of magnitude and it is the honest thing to point at.
Who actually needs one, and who does not. A cupboard in a heated British kitchen sits comfortably inside that range most of the year, and buying a meter to confirm it is a waste of eleven pounds. The places that fail are the ones this site’s readers actually use: a van, a boat, a shed, an unheated garage, a cellar, a caravan left standing over winter. USDA names a damp garage or basement as somewhere never to store shelf-stable food, and if that is where your food lives, a number is worth more than an opinion.
What to look for, since the cheap ones are all much the same object: a stated accuracy figure, usually plus or minus 3 to 5 percent relative humidity, which is ample for deciding whether somewhere is 45 percent or 75 percent; a combined temperature reading, because the two answers you want are the same two conditions; and a minimum and maximum memory, which is the feature that actually earns its place. A single reading at four in the afternoon tells you nothing about what happens overnight, and overnight is when a van gets cold and damp.
Ignore anything promising laboratory accuracy at this price. You are looking to tell a dry place from a damp one, not to calibrate anything.