mixed with keeping refineries the same (maybe a touch cheaper) and with getting the trade chain bonus of trade ports down ... can we increase the heights and lows that the black market will rise and fall too? This might prompt more players to go the refinery route is in a certain game items are beginning to become fairly expensive.
When the game first launched, the black market prices kept going up and up as people bought a resource without selling. Crystal was usually in high demand; in some games the price for it would go up past 1000 per 100 crystal (if memory serves?). The first thing everyone did when a game started was try to purchase 200 crystal because in a 10 player game the last guy to buy his crystal was paying twice as much as the first guy did.
The current black market is set up not to punish people too much if they can't find a specific resource. Because of the rock / paper / scissors relationship of ship combat, you would be completely screwed if your opponents 8 planet empire has balanced resources and yours was severely lacking in metal or (usually) crystal and you couldn't build the ships you needed.
Ages ago I suggested that there be an option at game setup to disable the black market, or make it a "closed" market where there wouldn't be anything available to buy that had not been sold into it. The devs did comment on it being interesting, but I'm pretty sure it didn't make it in because they don't want to create situations where people "game" the resource market to win.
