Okay, trading is apparently beyond my comprehension. I found a great route, dried it up and made ~4 million off of it. I've yet to come to anything even remotely close to 1500t round trip since. I go to high pop extraction sites, been looking for days. All the supplies are dried up. I'm looking at like, max 1100 margins.
Might jump back in a cobra and pirate some shit to break up the monotony. IDK. Mining? Shiiiiiit
There are very few high pop extraction systems, only 2 or 3 that I can think of, so I'm not sure what you are doing. There are high pop agriculture/industrial/etc combo systems with extraction/refinery stations though, these are what you want to look for Here's some tips that should help:
1) when looking at map view do not filter out systems by economy as it will filter out combo systems which are frequently the best locations. Adjust your population filter by lowering the max slider down 1-2 notches and raising the min filter around half way. That way you can see which systems are large pop and not even bother with low pop systems.
2) start by finding a large pop extraction or refinery system. Almost always these will be combo systems such as high tech/refining or agriculture/extraction. By large pop I mean systems with > 100 million population, preferably in the billions.
3) check out the system in the system view, make sure it has a station (not outpost) which is categorized as large or huge population refinery or extraction or combo. Preferably wealthy or in boom state if you can find one.
4) go to station, check to see if they have high value metals in high supply (with supply at least 50,000) and min prices (e.g. gold should be under 9000/t). If they don't have anything good, move on to another station in the same system or find a new system.
5) if they do have high supply and low priced precious metals, check to see what the station has demand for. Usually the best bets are consumer goods, progenitor cells, performance enhancers, or resonating separators. If they have a high demand for one of these, then buy some gold and on to step 6.
6) on galaxy map, look for high pop industry or high tech nearby to sell to. Use same method as step 1 although the constraints can be less strict depending on the quality of the station you are at now. Once a suitable system is found, buy their trade data and make sure they do import gold and export e.g. prog cells. Go there and set up your trade route.
7) if the destination turns out to be a dud (low supply, bad prices, etc) try another station (or alternatively to make some money, sell gold and go back to the first station and try again). If you can not manage to find a good 2nd station after several tries, move on and back to step 1.
Keep in mind combo systems such as high tech/refinery can either import or export gold or tech items depending on which economy is dominant so it can be tricky to find a suitable route sometimes. It usually takes me around an hour to find a good trade route.