Once I hit 46, I started mining darksteel ore and was the only provider for the whole server for about 5 days. So here are my prices:
3 DS ores on market: 13k each
And then ppl who bought your ore at 13k each go charge 9999999 Gil for their craft (at least they have to cover the mat cost) and since only Gil buyer can afford that,all the crafters who bought ore from you are filled with RMT gil. Of course they will keep buying your ore again, so eventual RMT flow back into your pocket.