This could work. The only thing is that it mandates using offworlds at a certain point. Price would also only ever rise offworld, which I still find bothersome. It will also mess with the game length. The exponential growth should tie in line with the desired game length, but im not sure offhand how long 30 minutes is in game days. By market prices, do you mean onworld? If so only glass/electronics would always be the end game resource as they start highest.