Ideally, I want most item stats to be variable in-game. On the most basic level, there's 'sword' with an attack power of 5, and then there's 'rusty sword' with an attack power of 4. This could happen during use though, probably during repair. If the sword hits, say, 0/15 durability, then you can repair it to 15/15, but it has a chance to lower it to 14/14 or to inflict a poor status, depending on your sharpening skill and the quality of the grindstone. In this way, items become more fluid. You don't have "a sword or no sword" you have "an old sword or you can spend your only money to get a shiny new sword" and other such item-specific issues.
The most "efficient" way to maybe do this would be to have item stats tied to durability...for example
sword at 50/50 durability does 5 damage (base damage x base durability....in this case 5.0 x 1.0)
sword at 40/50 durability does 4 damage (5.0 x 0.8 = 4)
a sword with 100/100 durability (skills modify the max durability of a weapon or item when crafted. 5.0 x 2.0 = 10)
With this system everything can revolve around "durability"...which could perhaps become "quality" or quality is simply the value given on creation (IE the max durability)
Regardless...the price, quality, and stats could all be tied to the durability stat which means you need to only set the base stats one time and have everything alter based on the current durability.
in short
base stats x (current durability/50) = end stat
/50 is of course assuming 50 durability is an "average" item with 100 being the cap for a "legendary" item
a slightly more complex system that could also work is
(base stats x quality) x (durability/50)
Initial quality (which sets max durability) would be determined by skills on creation. This could result in a poorly made sword that can be "sharpened" (increased max quality) but never truly made better without making a new one (max durability capped by initial quality value). Nothing of course would stop someone from "reinforcing" a weapon or "rebuilding" it to increase max durability.
Hope that makes sense...I am tired lol