There's no real guidelines in any rule books (to the best of my knowledge). Since any wizard wanting the spells would still have to scribe them him/herself, I would say it depends on the availability of the spells in the book, and spells in general. If there are free magical libraries, and only common spells in the book, then it's practically worthless. Rare spells would up the value, as would sources of spells (libraries or individuals) charging for access to spells.