A character goes to a bookseller and wants to buy Volo's guide to Monsters.
With support from your expert opinion, how would you price the book?
The bookseller has the book in stock, somewhere in the forgotten realms.
In the Tomb of Annihilation module, in the Chult region of the Forgotten Realms, players can buy a copy for 50 gp. However, it comes with limitations.
If the characters buy a copy of Volo’s book (50 gp for a durable hardcover edition), [...] Any time they want to know lore about a particular monster described in the book, give them useful tidbits from Volo’s Guide to Monsters. Do not impart game statistics, since such information would not be available in-world.
I have used the same price in other campaigns. If you carefully ensure to give out tidbits instead of abstract statistical information, it is of appropriate value. But I also cannot emphasise enough that you should even limit the bits in what kind of information they can convey. Make sure that it conveys only partial information at best. I approach it similar to collecting rumours, some of which are unreliable - to limit the meta gaming at the table.