The risk with having money in there though is that we get carried away with this idea, and say, so you can sell stuff you don't want for money, and buy stuff you need for money. This works in your example, but imagine:Starrh wrote: lets set up an example three worlds
LotsofFood wants production and is willing to give up food has no need of minerals
lotsofproduction wants minerals and is willing to give up production has no need for food
lotsofminerals wants food and is willing to give up minerals has no need for production
in this example cash can make this work very simply. lotsofproduction can sell product to lotsoffood for cash lotsoffood can then buy minerals from lotsofminerals. This is a simplistic example and of course the more complexes system will give you more flexibility. In my opinion money will conform to the kiss concept a little better, everyone is familiar with the use and exchange of cash and in the midst of a trading situation you do not have to search out a combination of trades to get what you want.
A) Loads of food. - has lots of food, needs minerals
B) Lots of Food - has some spare food, needs some production
C) Lots of production - willing to give up production, needs minerals.
We blithely convert stuff to money rather than sorting out the trades and get A sells food buys minerals, B sells food buys production, C sells production buys minerals. This works in terms of money, but of course we've just invented the minerals from nowhere, and sold more food than anyone wanted to buy! (this is a danger in any setup that isn't precisely balanced, i.e. every combination)
One alternative, properly netting off the surplusses and deficits and then trading them between planets rapidly leads you into headaches trying to model the correct prices of commodities when modelling the market economy, just ask Krikkitone, he and EntropyAvatar bashed their heads against this one for ages.
The other, hamstringing the conversion process with strange rules, limits and ad-hoc arrangements, makes the players life so complicated you'd be much better off just keeping the resources completely separate and handling the surplus and shortfalls by independant allocation rules.