Mowglia
Recruit
Your economics basically changes over time. Initially you will likely be getting gold from looter loot and/or tournaments/prizes.
After a while you can start bulk trading, supplemented by workshops. I don't want to get into the details (given they will invariably change) but I typically hit a capacity of about 50k carry weight. Don't ask me how many horses that is. I only use Sumpter Horses (freight), Desert Horses (troop movement), and Aserai Horses (me and my companions), although just use whatever works. I obtain most horses (and grain) from the southwestern Aserai city (Askar) or nearby.
Except this will only get you so far. The problem is that trade requires spending time moving/working for gold, and there's a cap on how much of that you can do. Moreover, you don't really have the time past a certain point (as some kingdoms might get too powerful)...
Next comes the difficult part. Personally I don't play as a vassal (ever), so can't comment on that. As a ruler you have to set up a kingdom, which is the key to unlocking income from settlements. Getting the first castle is hard, and by far the most difficult thing I experience in Bannerlord. Specifically, you're in a situation where you don't have many troops, you don't have many (if any) vassal clans, and even with trade your budget is likely to be tight. It's all about acquiring the men, and funding them. At this stage my income can go severely negative, so I save as much as I can beforehand.
Once you've got a castle, everything changes. You can form a kingdom, which means you can use an army and send companions out to recruit for you (and add them to your army when they've got enough men). You can now keep men in the garrison, which helps for defence and reserves, but also if you need to send companions out to recruit even more troops.
You might still be running a large deficit, but don't panic; your companions recruit men rapidly (far faster than the player). While doing this your companions also seem to boost local relations where they are at (not sure how), and this helps a lot with your own recruiting over the long term.
The next target is a city. Launch everything you have at it, even if it means sacrificing the castle. It's best to pick somewhere with good potential (ie., big/prosperous) and good access, but beggars can't be choosers so if push comes to shove just grab whatever you can get. Big cities are likely to be heavily defended.
Once you have a city (and hopefully still a castle) you should see a significant boost to your income. You also have a stash to store tens of thousands of grain and other food, which will be needed to feed a huge army (bearing in mind lords might tend to use your food while in your army). Personally at this stage I try to keep expanding using only my companions for as long as possible in order to retain the fiefs (or use a mod for keeping fiefs), although eventually you will need other clans to help supply troops and keep the enemy busy. The more settlements you control directly, the bigger your income.
Given you keep expanding and continue to accumulate settlements then you will reach the point where income from settlements dwarfs any income from trade. Trade and workshops become obsolete, although you will want to maintain a fairly large capacity (horses) for moving vast amounts of food and men around.
So don't worry too much about trade. Trade is merely an intermediate step before you move onto much bigger things (i.e., income from settlements).
