NQ White Capes are a quest reward from a quest that most people can do pretty easily, which explains the high supply relative to the demand and the low price. It has little or nothing to do with the cost of materials or the actions of clothcrafters. The price would probably be even lower if you couldn't NPC them for ~1400 gil.
You might think people would choose not to do a quest that requires a ~6k Mythril Ingot and rewards you with a ~2k White Cape, but of course profit isn't the only reason people do things in the game.
Crafters need to set the price steady on this. It has become such a loss synth that the White Cape +1 price is more than a Red Cape +1. Ask at least 5 or 6k for this, if a WHM doesn't have 5 or 6k gil by level 32 they shouldn't be playing FFXI.
