How much does a book cost?
The room I’m in right now contains approximately 1800 books. The shelving on which those books (along with not very much else) are stored cost, I think, about £1500 or so. (You can get bookshelves that cost a lot less, if you don’t mind them sagging and/or looking unsightly. You can get bookshelves that cost a lot more, if you want beauty as well as quality.) So, crudely: shelving a book costs £1.
That room, though. It didn’t come for free, and if we had drastically more books we’d need a bigger house to put them in. (I’m fairly sure that every time we’ve been through the soul-destroying business of buying a house we’ve rejected some houses on the grounds that they didn’t have enough spare wall-space for bookshelves, so this isn’t a purely theoretical concern.) A quick look at house prices in our area suggests that one decent-sized room typically adds something of the order of £40,000 to the price of a house. (Less for smaller houses, more for larger houses; larger houses have larger rooms.) So, crudely: housing a book costs £20.
I’m not sure I wanted to know that.
Of course there’s lots wrong with that analysis. For instance, this room contains not only books but also computers, desk space, etc., and we can fit quite a lot more books into our house before it’s so full we have to buy a new one. Even so, I think it would be quite difficult to justify an estimate of the overhead cost per book that’s below, let’s say, £5. But do I think of that in second-hand bookshops? Why, no, I do not.
There is presumably a broader lesson here.