OK, I can accept this explanation, although I find the idea of using anything but the highest-quality paper in a spellbook, of all things, to be a little odd. Maybe spell inscription inks would be cheaper if they just had acid-free!
Here, though, the proposed hack is more an absurdity to demonstrate that a spiked chain weapon probably shouldn't be cheaper than a regular chain, whether or not you can actually convert them in the suggested manner. I don't know a lot about forging chains by hand, but I do know enough to be fairly sure forging chains with extra spikes and hand-grips is more complex and more expensive, even if only by a little. So if they were priced the same, it be probably be OK, or if the weapon is more expensive, but cheaper? Not so much.Spiked Chain: That doesn't even work, considering a spiked chain is certainly shorter than 10 feet, and is not simply a straight length of chain (see illustration on 119 PHB for an example). Also, removing the spikes wouldn't exactly be a simple matter. How do you 'remove spikes' when the spikes aren't an add-on, but part of the forging of the chain?