You're missing the point though, the objective of the game is not to have something to spend money on, for my playthroughs its something like this:
Early game:
1. Find tournaments, I've noticed they give horses while you are clan tier 0-1, which sell for around 8-14k each.
2. After getting around 150-300k denars, gear up, meaning get recruits, companions and equipment.
3. Look for a clan to become mercenary, reason not being money but having somewhere to level up skills.
Mid game:
1. After some mercenary work, upgrade gear, upgrade units, level up myself and companions.
2. At around clan tier 4-5 , join a kingdom, deny all castles given until you are given a town.
3. Make an untouchable bastion out of said fief with the leveled up brother, that is usually done by doing certain quests which give prosperity after finishing them and leveling governors engineering skill.
4. Last bit of gear upgrade, tickle in whatever last bit of experience/trait I'm missing.
Late game:
1. Paint the map your color.
Now as you can see these are objectives throughout the gameplay loop, and it gets tedious because its repetitive and pointless doing the same thing over and over. Now you might say there is criminal enterprise you can do, or be a merchant with caravans and workshops, etc. The thing is whatever else idea/objective you might have, it is either broken, half implemented or pointless. For example criminal alleys are half implemented as they are nothing more than a prosperity based income, which we all know towns starve themselves to death day 1, workshops don't have upgrades, income is low due to the economy logic there is, also traits that improve workshops actually make them worse, caravans are pointless, no traits to improve them (carry weight in Trade skill tree did nothing for the profits), no way of upgrading them with better mules, or units, I know there is an option to pay for better units but its just 29 "better" geared dudes who still die no matter what, what if I wanted to upgrade to 50 or 80 people in one caravan.
TL;DR
In a RPG/Sandbox style of a game the objective is not linear, it progresses with the story. Linear type games: Cookie clicker, the point being just click to get cookies and spend cookies to make even more cookies so you can spend those again!
So in Bannerlord you will get flooded on denars, influence, war bands running around to help/fight you and they become irrelevant, so you don't need a reason to go back to spending money, you need a reason to look forward to a new objective.