I would suggest missuse's Item Price List to find how much any item is worth.
"Platinum f___________________________25-45k
Supernova l_________________________10-20k"
All the platinum forces that went through my hands were bought for 40k so that's why I'm saying you wouldn't pay more than this ^^. But it depends on weather or not you are willing to wait for it or not. If you want it badly then pay as much as the seller wants for it.
@hycx, hunter force is sold out and we don't pay 50k for it? Same with platinum force. And also, platinum isn't usually as popular as pure and quicksilver, making it slightly harder to sell..