Since your armor takes the brunt of damage in battle, you’re in constant need of repair. It’s BB’s main money sink, really. Ammunition depends on how often you’re using ranged attacks, and even the medical supplies for me drain pretty slowly (I tend to hit temples asap if my men are injured, though). Food and tools are the 2 major sinks.
What annoys me is that regardless of relationship w/ faction or town, markets will only give you a fraction of the value of goods you are selling (talking weapons/armor, and not trade goods). I understand it’s not going to be anywhere near the actual ‘value’ of the items, but they give you less than 10% of the value of the item. Often much less. That makes keeping up on payroll, food, and tool costs a real challenge in the early game depending on the initial setup.