So, right now the Black Market is an infinate well of resources, with a slight reaction to trade volume, a little volatility... but basically just noise on top of buying resources for 4.5, selling for 2.5.
What if the market was really a market.. a closed system for resources?
Have all the independent planets producing resources and selling continuously, according to planetary resources available. This would ultimately limit supply. As players take over the map, and no independent systems remain, the supply would be limited to what players are producing and actually selling. It would tighten things up.
The system attempts to adjust prices up and down to maintain a steady volume of transactions. Players buy, prices go up. If the amount the "market" has on hand is over a certain amount, prices drop until they equalize. If the amount on hand is low, prices go up until it equalizes. There could continue to be a "markup" amount between buy and sell -- this would eat up the extra credits that the planets are constantly generating and help control inflation.
The system could keep track of the total number of credits in circulation, and adjust the "markup" accordingly. The market would be an entity that attempts to control the amount of resource in its possesion, and the number of credits in circulation. All trade passes through it, so there is no need for players to use the bid system if they don't want to. Just buy and sell, and let the market do the work.
What you would get is genuine and realistic economics. The best part... the interface doesn't change AT ALL... everything is behind the scenes. Maybe add a volume window to show the amount of each item the "market" entity has on hand. Despite this lack of interface change... a whole new element of strategy opens up. The game gets less RTS like and more 4x like.
Control 80% of the crystal mines out there... well... the price goes sky high and players have a hard time buying it. Use your crystal markets as a cash cow, or restrict availability? Your choice. Certainly opens up some diplomatic options. Resources are interchangable still, but only to a certain extent. You have to think before dumping all that metal on the market... will it get bought up and used against you? Maybe you have tons of credits. Do you buy a huge amount of metal to restrict other players from getting it, or to drive up the price?
We just found a new use for LF's (they don't need crystal)... you could even have a couple new ship types with similar stats but different resource requirements. Also, maybe producing a unit while short on crystal reduces its shields, short on metal... reduces its hull. Techs to control the magnitude of this effect.
Totally changes the port/refinery situation. Much more dynamic decision. If everyone builds ports, we see an excess of credits in circulation, the prices of resources goes up... so refineries get favored. If everyone builds refineries, resource prices drop... now people need to get credits directly with pop and trade improvements, since EVERYTHING has a base credit cost. Since all three resources are directly useful, they simply fluctuate in value against eachother.
Also, you don't build your economy just to spam ships and techs... after all, if you burn lots of resources, the prices go sky high. Makes the unit quantity/quality trade off more dynamic strategically. Materials are expensive... well, then tech up to make more efficient use of them... materials are cheap? Well, build massive fleats. To enhacne this, it is even conceivable that only credits (or mostly credits) get used for research instead of material (afterall, information economies use fewer raw materials, more labor == credits).
Implimentation would be a simple control system.. programming is easy -- you'd have factors for markup, inflation, and price elasticity of metal and crystal. Tune those four numbers, and you should get a stable system.
Think of the diplomacy factor... you don't want to tear down that militarily weak economic empire, because destroying his mines hurts everyone, the supply of everything gets affected. Instead, you take him under your wing and protect him, in exchange for him transfering metal to you instead of dumping on the market. It's cheaper than killing him, taking territory, and rebuilding all that infrastructure. Alternatively, maybe you have a good supply of crystal mines, and want to take out a militeristic empire... but rathat than attacking him directly, you attack someone else, just to tear up his mines, to restrict your enemy. There would be genuine REASONS for diplomacy. Nothing artificial. Also, there would be balance between war and peace -- just like in the real world, where tearing down your neighbor's economy has a cost that must be weighed. Players even have some incentive to prevent expansion into neutral worlds that they aren't currently prepared to redevelop... you could sign peace treaties with individual neutral worlds.
I emphasize again that you get all this additional strategic depth with no additional player complexity -- the existing market window and an explaination of what is going on underneath it suffice.