As in most recent wars, military casualties on the US-led side have been minimal in the war on Iraq. But there is another sense in which the cost in American, British and Australian lives has been high.
The money spent on the war could have been allocated instead to improved health care or public safety measures. The cost-effectiveness of such measures varies a lot, but a fair rule of thumb is that at the margin, health interventions cost about $US5 million per life saved (because of lower costs here, a marginal cost of about $A5 million per life saved is also appropriate for Australia).
So if we assume that the war and occupation will end up costing $100 billion, the opportunity cost is around 20 000 American lives. Assuming Australia spends $1 billion, the opportunity cost here is around 200 lives.
To take a more optimistic view of the question, it’s worth noting that the cost of saving lives through health care and other interventions is far smaller in poor countries like Iraq than in rich countries. So, if the US were prepared to spend another $100 billion* rebuilding (as opposed to occupying) Iraq, and allocate a substantial portion of that to health and education [especially education for women, which has big health benefits], many more than 20 000 lives could be saved. A commitment to reconstruction spending on this scale would go a long way towards justifying the war.
*Of course, for any net benefits to be realised, the money has to be new money from America or other donors, not Iraqi money recycled through American contractors.