Cepheiden

Slight Necro, but I don't know where else to ask: How to value +weapon damage? Wouldn't +1 weapon damag equal 14 ap? Wouldn't that mean, that the +5 dmg enchant is worth 70ap, which is much more valuable than 15 agi or crusader? Wouldn't that mean, the best ring in the game would be the might of cenarius with its +4 weapon damage?