VoinianAmbassodor had the right idea here. You need to take into account how far you are travelling and how much time is passing when thinking about trade routes. For example, let's assume that you have a fleet of 6 Miranu Heavy Freighters hauling cargo. If you used the weapons trade route that is the original focus of this thread, you could make 6.075M creds in a single run (that's your gross, not counting what you pay your Freighters). But this run would take a grand total of 42 days (assuming you only refuel twice in each direction). If you had spent the same number of days running medical goods between Atlantic Station and New Calcutta, you could have made 10.5 runs. Assuming the same fleet of Heavy Freighters, you would gross 7.0875M. These are both pretty good runs, but there are better.
The easiest way to calculate a route's profitability is to figure your profit per ton per day passed. This includes time passed for landings and jumps. This is crucial not only because of the playing time involved, but also because your fleet costs you more for every day that passes. Those six Heavy Freighter escorts bill you 72,000 credits every day, so you better have a plan.
To give you an idea using the Omm/Telnan/Pax route: buy meds low, sell high for 405 creds/ton profit; buy equip medium, sell high for 120 c/t profit; buy crassworms low, sell high for 540 c/t profit. The total profit is 1065 creds for every ton. Divide by 15 days to complete the round trip (assuming you make the 6 jumps from Pax back to Omm without having to refuel) and you get 71 creds/ton/day. (Or 66.7 if you need to refuel). This makes this one of the most profitable routes available. By comparison, the weapons trade route is only worth 48.2 creds/ton/day. In the 42 days it would take for the weapons route, you would gross just under 9M on the Omm/Telnan/Pax route.
So just how much are some routes worth?
Fertiliser to Centauri: 22.5 c/t/d
Food from Centauri to Earth: 7.5 creds/ton/day
plus 18 if industrials get sold on Centauri, or 30 if it's equipment (can't remember)
Colonists from Meria to Verril Prime: 28.125 c/t/d
Tourists to New Rome, Food to Earth: 36.7 c/t/d
Weapons to Emalgha: 48.2 c/t/d
Medicals from Atlantic St. to N. Calcutta: 56.25 c/t/d
Medicals from Omm to Thehir (in Hatuli): 67.5 c/t/d
Omm/Telnan/Pax route: 71 c/t/d
Miser's Funeral (Hatuli metal/equipment route): 162 creds/ton/day!
