The reason those things are expensive is 1) people who need them REALLY need them and are usually willing to pay, and 2) they're pretty annoying to make. You need items that only come from desynth, so even just having a level 50 crafter isn't enough if you want to make them yourself. You also need desynth in each craft at level 50 or so, and then the time and money spent desynthing however many items until you get the crafting mat.