In volume a paperback is actually about 50 cents. Excluding storage and shipping. Obviously even a homing pigeon is able to deliver a micro-SD Card and regular postage is less than a book now that special book postage rates are gone in most countries. A Print-On-Demand (POD) paperback is maybe twice to three times the cost of a cheap SD card.
I've no idea what the volume cost of a blank SD card is. Likely much more. Nor do I know what sort of production copying and testing exists, though I imagine it does. As a blank micro-SD card is a competitive mass produced item the margin on the prices of Chinese sellers may not be much.
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