There is an economic shift in the situation you describe which makes it unlikely. If RMT are selling 5 million gil for $20 then someone selling the $40 (platinum) ring will be asking for 10+ million gil. If RMTs then start selling 10 million for $20 then people start asking for 20 million gil for the ring. This will continue until it is unprofitable for the RMTs. This is a fairly effective way to mess up RMT profitability.