Yes 300k is steep, i'll admit that. Though it will not stay at that price unless it's in very high demand (which i doubt considering that you can just buy NQ or get dungeon drop / quest reward gear). As the OP has pointed out people will always undercut where possible. Often I'll see an item selling for triple the price and go ahead trying to sell at a slightly lower price. By the next day I haven't, there are dozens of people undercutting me. In fact so many people try to do the same thing that there's then an abundance of the item and not enough buyers, so it'll end up selling for less than the cost of the mats. This'll be either because there are higher level crafters with all cross class abilites that can make the item from cheaper NQ mats, or the people selling gathered their own mats and don't realise they could have made more by selling the mats themselves, or that they want to just cut losses and not hold onto an item that's value is plummeting.
Let me re-itterate though that it costs millions of gil to level up a crafter (or a lot of time), and I don't see why you would then be concerned about the buyers pocket. It's not like anyone buying BLM gear for levelling is required to do so. They have alternatives to gear up (dungeon drops, quests, NQ vendor gear). It could easily be argued that the BLM is the greedy one for wanting crafters to voluntarily cut the price on an item that they could sell at a higher price, for no apparant reason other than to make some other player they have never met have overpowered gear with less effort.
Yeah that's true also. Alot of that gear will be the unwanted quest rewards players have obtained with their DoW/M, while progressing story quests. But again, I see no reason why they should not sell at as high a price as someone else is willing to pay. Nobody needs to be buying level50-60 DoW/M gear to level - except for CRAFTERS! lol