The cost of a war is rated in percentages of base income, and is derived from the distance the war is being fought at and the level of of a fief's commitment.
Percent Cost Commitment Level Description 1% Minor Very small expeditionary force. 3% Major Significant force 5% Full Vast majority of forces
The distance matters, too. For a Short Distance conflict - in the boundaries of a fief or someplace immediately adjacent - cost is above. A Medium Distance conflict (within the same region) doubles cost, and a Long Distance conflict (cross-continental) triples cost.