Just because an item is for sale at a price that seems unfair does not make the sellers RMT. There is very little to possibly no RMT activity in Mt. Zhayolm or Halvung (never was any in halvung) anymore. I was able to sell ~12 of these ranging prices from 30k-90k. I was in Mt. Zhayolm immediately after the update mining for khroma ore, my usual occupation, now that so many people are mining for plumbago it's impossible to get a decent amount of mining points per hour. Because mining points are so hard to get now, prices will naturally be high... If you want your plumbago cheap go and mine it for yourself, against the 12-20 other people competing for it at the same time.
Another option is you could just be patient, eventually these prices will fall because they're easy to mine in Mt. Zhayolm and people are already there mining for khroma and other materials.
Considering it's been discovered that it drops from mobs now and doesn't necessarily have to be mined, if you're still paying more than 5k for this, you're a *** idiot.