Wars used to consume lots of lives. The civil war, WW I -- incomprehensible volumes of lives. Raw human life, however, is not all that expensive. I shan't belabor the point since it's grossly offensive, but in low tech societies like WW I Europe it didn't cost much to raise a man to die in Verdun.
Wars are getting more expensive though - even for the aggressor. Fighers are educated and their economic output is far, far higher than in WW I. Their lifelong post-war care costs more. As we substitute technology for fighters that costs even more. The medium term costs of the "small" war in Iraq to the US are reaching above 200 billion.. The longer term direct costs for the US alone are now speculated to exceed 1 trillion..
Of course expense is relative. As a percentage of an 6-8 trillion dollar economy perhaps a 1 trillion dollar war is still a "small war". Is war really more expensive for 21st century America compared to 19th century Britain? I certainly hope war is becoming more costly in relative as well as absolute terms, but I don't know.