a marginal devaluation , between 5 and 10% of the market value.
vintage 1965 strat needs fret job and a new nut. Will this impact the value? Should the nut be replaced, shimmed, etc.?
Devaluation from the value in its unplayable condition?(!)
Of course, value depends on the buyer- a guitar with nut and frets worn enough to cause playability issues will be likely to have other wear issues (finish, dings, etc.) that will also affect valuation to a collector oriented to a "mint is best" frame of reference, while to a player it might be a whole 'nuther thing-
A worn guitar with worn frets/nut, a worn guitar with well-restored nut/frets, and a mint guitar with replaced frets are three different animals, and present three different valuation issues-