Its slightly more complicated than that. As that makes it so anything that used to cost, say... 100 gil, if still listed at 100 gil, is a 90% increase in price. The problem lies in the fact that, those used to the current pricing will NOT begin charging 90% less for all their items. An item that sells in the MW for 10k, is NOT going to drop to 1k. It will remain at 10k. So beyond NPC values (which hold extremely little control over the economy), all they're really doing is making everyone poorer.