These graphs from Matthew Yglesias display the inflation-adjusted costs of the various wars the US has engaged in, both in absolute terms and relative to the size of the GDP at the time. The latter graph is particularly illuminating regarding how little, in relative terms, we have spent on Iraq or Afghanistan.
Graphs of casualties, either absolute or relative to population, would make the same point even more strongly. US deaths in Iraq number about 4,000 for a war that has now lasted over half a decade. In World War 2, according to Wikipedia, the US lost over 1,400 killed just in securing the beaches at Normandy and over 6,800 in securing Iwo Jima, one small island in the Pacific.
This does not mean that the Iraq war was worth doing or was not worth doing. But it does offer up some perspective and I think it helps to account for how quickly Iraq has faded from the radar screen now that the situation has stabilized and we have begun to pull out.