On the subject of trade, etc., one thing that has always been a failure
for me with sf games is that they mostly ignore the real impact of space
industrialization. Even if only one percent of a planetary population
likes being in space, what will happen in the long run is that their
children will grow up thinking of living in habitats as natural, so they
will expand into space rather than go back to a planet.
The resource base is pretty simple. A fully developed solar system could
have five or ten billion people on a planet, but hundreds of billions living
in habitats near asteroids, around gas giants, or in the cometary halo.
The "geography" of such solar systems would resemble an ocean full of
archipelagos, hundreds of large groups of habitats, each centered around a
resource base like an asteroid. Each archipelago might have thousands of
individual habitats (L5-type city-states with anywhere from ten thousand
to ten million people apiece...my own estimates are a low end of 40,000
and a high end of 1,000,000, but you could build larger or smaller and
get away with it). Trade in such a society would be limited by the
demands of physics. Fusion-powered spacecraft will be very cheap once
we get fusion under control (matter-antimatter is more fuel efficient,
but with fusion the fuel is hydrogen, which is very plentiful)...well,
cheap in relative terms, spacecraft will still be expensive, but fusion
drives will be very efficent. For most ship designs, most of an
inner solar system (Earth-to-Ceres, etc.) will be a matter of a week
or two of travel (assuming you don't use jump drives for intrasystem
travel) and a few weeks to the outer system (Earth-to-Neptune, etc.)
and several months to several years for parts of the cometary halo.
In other words, sailing ships of the eighteenth and nineteenth centuries,
and, to some extent, steamers of the nineteenth and twentieth centuries.
And a society that would resemble, in some ways, Europe, if it were
broken up into physically separate power blocs...or what the world
might be like the centers of culture were as divided as they are now
(Europe, America, China, etc.) but we only had 1800-1900 sailing/steaming
technology for travel.
By the way, I use a figure of 1 million for the Isp for fusion drives
(about what the Daedelus project people suggested), and 20 million for
matter-antimatter drives (which is a bit more than what Charles Sheffield
suggests, but easier to deal with in games terms than the 18 million he
proposed). (Much easier to do the numbers this way, and it does not
affect the realism of the game in any significant manner.) The average
spacecraft in my game world uses 30% of its mass for fuel, which, at
one gravity, gives it 352,000 seconds of thrust, which means spacecraft
will go to about 1,600 km./sec., and then coast at about one billion km./
week until they are ready to slow down, and then slow down (this leaves
a 10% reserve of fuel for mid-course corrections, etc.)
I will admit to vanity when I say this, but I really like the way I have
worked this out because it means that every single fully developed solar
system (Tech 11 or Tech 12 or more) is an enormously exciting place, with
the equivelent of dozens or even hundreds of worlds in every solar system.
Imagine, fellow GMs, the kind of intrigues you can have, with dozens of
powerful governments and megacorporations vying for influence, vying to
control the lives of half-a-trillion people, and this is what goes on in
every single solar system in your game world. (And, if you like complex
and paranoia-inducing plots, remember that at this level of complexity,
you have governments and corporations so complex, and with so many levels
in their hierarchy, that different branches will be trying to screw each
other over...)
Enjoy.