It seems to me a very stupid guideline because, while it makes economic sense that an item usable by more people would sell for more as there's a greater pool of demand, the gold price of an item is the basis for a bunch of other things that shouldn't be effected. Either a character can or can't use it, so he's not getting one-third the benefit, he's getting all the benefit or none. Nor does it make sense that it would take only one-third the time or money to craft.

The most consideration I'd give to that guideline would be to calculate the GP value normally but advise the player that if he wants to sell it he'll get well below normal market price. This can be used to encourage players to hold on to items tailored to them. For most purposes you can probably safely disregard it entirely.