This used to infuriate me to know end until I picked it apart. I'll break it down with some references to the above post.
Let's say you have the same 3000 florins. This is all the money you have to begin the construction of a building or a unit. Buildings or units that have already started being built don't count. You pay for the total cost for a particular thing up front.
You have queued up to start construction of 3 keeps and a watch tower. When you hit the 'end year' button the game spends all the money you have on hand at that moment, the 3000 florins. You are spending 3100 but you only have 3000. Well the computer is going to start construction on 3 of the 4 things and 1 of those things is not going to be built. I am not sure how the game decides what order to build things but the provence that is constantly getting short-changed is on the bottom of the game's list of who gets what first.
After it starts the construction of what it can then it collects the taxes for the next year. That's why it seems like you aren't running out of money. In the above scenario the 'insufficient funds' message will come up, it will not have started construction on a keep (for instance), but you will have another 3000 to spend next year.
Two tricks I use to avoid this situation is:
1. Keep a tally of all the things you are going to build, or at least a rough estimate. Make sure that sum is less than the money you currently have.
2. Don't queue up a list of things to be built. That way you don't end up accidentally overspending for something. When the message comes up that you built something, or a series of somethings then you know you can 'go shopping' for more stuff. This one is harder for units considering the game does not tell you that you built a unit of archers. You will have to tweak the unit thing yourself.
There are others such as, don't expand to rapidly as early tech buildings get built quickly and tend to suck up cash quickly, across 5 or 6 provences this could break the bank. Decide what your building where. Remember that every one of your Provences do not have to have every single building and they don't have to build all the time. Take Flanders the Provence has great farming and great trade, and maybe even mining. Try to build its monetary potential, its military potential should take a back seat, it has no regional specific units or valor gaining units to benefit. If you have maxed out farms, trade and mining in Flanders and you have just enough cash to build a bowyer there or in a Provence closer to your front lines then build closer to your front lines where you will be focusing most of your attention.
The above paragraph you don't have to adhere to. Many players have different playing styles, objectives, or needs. The above needs to be tailored to suit what you want to achieve, I'm just throwing out food for thought. If this post was a little long-winded its only because it took me so long to figure out a system from keeping myself for the same predicament you are in now. It was very frustrating for me at first. Remember its much easier to pummel the AI into submission than it is to build an Empire. Good luck and more importantly have fun.
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