what's the formula for determining the cost of a (non-magical) item that gives a competence bonus to a skill?
for example, there are numerous examples of libraries that give a competence bonus to one knowledge skill. what would be the value of such a library?
I don't think there is a generic price, or if there is one I don't know of it.
Artisans Tools: 5gp Masterwork: 55gp Difference: 50 gp
Thieves Tools: 30gp Masterwork: 100gp Difference: 70 gp
Musical Intrument: 5gp Masterwork: 95gp Difference: 95gp.
A library would be particularly hard to figure out price wise. While it may be that it contains one particularly good book on the subject at hand, it would be equally possible that it has several books that have enough 'hints' that you are able to get an answer. Of course, many many of the books in a library won't have any bearing on a particular question, but might apply to others. Personally I would expect a generic library to cost many hundreds or even thousands of gold. A more focused book would cost much less, for example +2 on Knowledge Nobility for a particular nation for the last 100 years I could see being just a few gold.