How does one estimate the costs of an armed conflict?

There have always been eye-catching headlines about conflicts regarding their cost, e.g.

• The war in Iraq has cost USA x billion USD
• The US presence in Afghanistan has cost the exchequer y billion USD

How does one estimate a cost of a conflict? Should it always be an estimate? Is it possible to get an exact cost, like one does for any venture?

• Is it possible to have profitable conflicts as opposed to loss-making conflicts?
  • The Empire left their colonies post World War 2, because they did not find it profitable to maintain a presence after their World War 2 losses.
  • Afghanistan was a loss-making conflict, as opposed to the profitable Iraq conflict
• Is there a standard way of calculating the costs of a conflict, using double entry book-keeping accounting methods?
• What are the tangible and intangible assets in a war?
• How does one calculate reduction in the value of intangible assets of a war as they may not be immediately manifest?
• Are the assets owned by Armed Forces subject to depreciation or appreciation? If yes, how does one calculate that?
• Just like before embarking on a business venture, a firm calculates the profitability or, at the very least, sustainability of the said venture, do nations calculate the profitability or sustainability of an invasion/occupation?