Supply and demand?
I know Filip said prices are based on players' actions. What I've not heard is exactly in which way and it would be great if manual mining and refining which takes time and PCUs was still necessary. People seem to already assume local prices on planets and moons and I'd find it great if players doing mining or trading would be needed to supply planetary markets (delivery to any station will do) with ingots that are in high demand to keep the prices on that planet in check. (Does need to mean planets run out of platinum entirely, just have it very expensive.)