In my game that is stuck in a fief designation crash cycle now I was easily making 10k per day, and extra through my own sales, AFTER maintenance for several 100+ garrisons (of cheap units admittedly) but a personal army of 200ish top elite troops (and a few odds and ends in training to replace the few losses sustained).
I had no workshops as I forgot to buy any.
I had 3 trade caravans. The trick is assaulting lucrative places and focusing on building them up.
First city I took was revyl because its super easy to defend that peninsular (only one way in or out) few k for peace with the sturgians. Got food/trade sorted out the went after the expensive nice cities from the war torn empire factions, Epicotrea, Diathma, argoros (argoron?... the city east of Diathma). I think I personally owned rhesus castle and the one behind Revyl too.
Anyway. Land ownership. That's how you make pots of money. Their income and any war/loot income resulted in over 1.5million at one point.