Note that Fringe Space is for playing a few ships in a Campaign.... you're not the grand admiral of Earth's fleet. The economics system reflects this... often times a planet is no good to trade with because the big merchants and government aid, which is obviously beneficial to them in some way, make it so that the market isn't good for a small trader.
So worlds are defined (economically) by three factors:
Type: A vague description of the world's place in the economy.
Barren: These worlds don't produce anything valuable enough to trade in space (at least not in bulk).
Farm: These worlds produce an excess supply of food and similar necessities, while being relatively weak in other areas.
Mining: Produce useful materials and even low level manufacturing, but are net importers of food and more complicated machinery.
Generator: Similar to Mining worlds, but the specific material that they produce in abundance is an important source of energy.... a good place to refuel your ship! (note that this is often "artificial" energy... its not that there happens to be rocket fuel sitting around, its that this planet has a lot of infrastructure built for, say, mass producing antimatter).
Developed: Like Earth. Combines the food of a Farm planet with the resources/manufacturing ability of a Mining planet, while also having the infrastructure to produce complex machinery (like space ship parts). Also an important source of Luxury goods, as these planets tend to have a longer history and larger culture. Doesn't generally import, with the exception of luxury goods.
Trade: Kinda like the opposite of a Developed planet. While a Developed planet is essentially independent, a Trade planet is useful more for astrographic reasons than its natural resources or infrastructure. For whatever reason, this world is a common stopping place for merchants and so has very fluctuating market... it rarely wants (as there is a constant stream of goods from larger corporate trade) but also frequently doesn't offer great prices for a buyer (as a lot of stuff is imported). On occasion great profits can be had buying OR selling from these worlds, though, and these worlds are also a center for intrigue and adventure. Fuel tends to be an important import.
Equally important to World Type is Population and Law Level, which both range from 1-6. Population has to do with raw population as well as density, and essentially shows purchasing power as well. Law Level basically increases buy price and decreases sell price because you need to get around tariffs or bribe officials to sell goods. This can be negated by smuggling, but this can be risky.
The Goods are:
Material (Usually from Mining Worlds)
Machinery (Developed Worlds)
Fuel (for your ship OR to be traded, though this is rarely profitable)
Luxury (which come from all worlds).
There are also special trade items but those are specific to adventures. (for example, a rare alien artifact, or supplying weapons to a rebellion).
The buy and sell price for these is modified by planet type, population, law level, and sometimes even distance from port of origin (luxury items are more valuable from farther away... what self respecting 3rd world dictator gives his guests French wine when he can get the Martian crop of 2128? For this reason Luxury goods are marked in the cargo hold not as "luxury" but as "Mars-Luxury")
All goods are available on all planets, but in some cases there is no way to get a profit. Keep in mind that all listed profits in the following exercises do not include travel expenses, like fuel. Under the current system it looks like you need to be careful with planning your trade routes, especially if you are mobile. There are some cases where you can literally fly around one world and act as a cargo airplane of sorts, but these tend to be the sort of worlds you don't want to stay on too long... or you can only buy and sell a little bit in any given week as the population is low.
When selling OR buying you are always limited by the world's population... you can never sell more units than the world has population in one week (technically a quarter month, the campaign turn), and you can never buy more than twice the population.
First I set up my little system:
Earth (Grav 3, Developed, Population 6, Law 5) - Luna (Generator, Grav 0, Population 2, Law 2) - Orbital Stations (Grav 0, Barren, Population 5, Law 3)
Mars (Grav 1, Trade, Population 3, Law 3) - Mars Orbital Stations (Grav 0, Farm, Population 1, Law 3)
Asteroid Belt (Grav 0, Mining, Population 2, Law 3)
Some of this was randomly generated, but a lot of it wasn't. Doesn't matter too much which you do.
Here's how prices are determined (In "Items" from New Beginnings):
All prices are modified in the following way
1) roll a d6
2) if you're on a trade world, then on a roll of 1-2 subtract 2 or 4 (1 and 2 respectively) from the buy price and increase the sell price by the same amount. If you roll a 5-6 do the opposite.
3) If you're not on a trade world, then a roll of 1 will decrease buy price by 1 and increase sell price by 1, and 6 will do the opposite.
Note that it never costs money to sell something and you never make money by buying something. All negatives become 0 instead (so cheap its basically free). If, for whatever reason, you can sit around on one planet and buy and sell the same good and make money (for example, on a mining world with a population of 1 and a law of 1, then material costs 1 but sells for 3) then you can do this, however, you must wait a turn to sell after buying... you'll need to pay all expenses for this time. This represents you moving goods around on the planet itself and making a small profit. You'll still need to pay for the fuel of flying around the world, as well. (this is usually only possible with high population planets, so resource disparities makes sense. Also, low Law... so you're also in increased danger on the planet of being pirated).
D= Distance (1= same orbit, 2= one orbit away, 3= two orbits away, and so on...)
In the formulas you'll note that sometimes a variable is marked in the following way:
(P:x). This means that the number there is the difference between the given world's P value and a P value of x. For example, if (P:3) is seen in a formula, then if the world's P=5, then the number used is 2. If P=1, then the number used would be 2 as well.
Always round up fractions.
Barren 7+L 7+L 7+L 3+L -P+6
Farm (P:3)+L 4+(P:4)+L 5+(P:4)+L 3+L -P+6
Generator 4+L+.5P 4+L+.5P 3+L+.5P L -P+6
Mining 4+L+.5P (P:3)+L 4+(P:4)+L 3+L -P+6
Trade .5P+L+3 .5P+L+2 (P:3)+L+1 4+L -P+5
Developed .5P+L+2 .5P+L+1 (P:4)+L-2 3+L -P+4
Barren 8+P-L 8+P-L 8+P-L 6-L 3*D-L+.5P
Farm .5P-L 8+P-L 10+P-L 6-L 3*D-L+.5P
Generator 8+P-L 8+P-L 10+P-L 0 3*D-L+.5P
Mining 8+P-L .5P-L 10+P-L 6-L 3*D-L+.5P
Trade P-L+1 P-L+1 (P:3)-L+5 8-L 3*D-L+P
Developed (P-L) (P-L) (P-L) 6-L 3*D-L+P
Alright, enough of the scary looking formulas (they're actually quite simple, of course).
Here's my analysis:
The best places to buy (not taking into account gravity and position, though because I based the planet type in part on gravity thats in many ways accounted for already):
Food from Mars Orbit (honorable mention to Moon)
Material from the Belt (honorable mention to Moon)
Machinery from Mars (honorable mention to Moon, Earth)
Fuel from the Moon
Luxuries from Earth (though because luxuries depend so much on where you're going for their value, and the fact that they're all relatively cheap on the home world but expensive over at the next orbit, they're all pretty good).
The best places to sell (not counting luxuries because I'm too lazy to do an analysis from each planet to planet.... generally its best to sell at Mars or Earth and be selling from as far away as possible. Earth--Belt and vice versa is one of the most lucrative trades in the setting, but its costly in fuel and time).
Food in Earth Orbit (honorable mention to Moon, Belt.... note that the moon has multiple cases where you can do intralunar trade and make a nice profit.... its just that the moon is low population AND law, so its a bit dangerous and you can't trade much at any one time, and you generally don't make as much).
Material in Earth Orbit (honorable mention to Mars Orbit, the Moon)
Machinery in Earth Orbit or the Moon (honorable mention to Mars Orbit and the Belt)
Fuel on Mars (everywhere else you are at a net loss, at least when you account for the fact that you need to get where you are going, too).
So there you have it.
I think it sets up some interesting situations, and once you get going its not overly complicated.
Of course the problem is that if someone is generating more than just one system (note that I'm ignoring unsettled planets which normally you don't, though for trade purposes you do, and that I'm also doing just half a star system.... the outer planets are very active in most settings with all the little moons) it can take awhile to work out the trade routes... but then again, one can generally tell a good trade just by world type. Its just that the hard core merchants who really want to get into it can also take into account gravity, travel time, etc, and figure out the best route....
So its a good enough system that a mercenary can do a little trading on the side with ease, but also that more detail oriented players can really make good use of, IMO.