The report on Wednesday, entitled The Iraq Quagmire from the Institute for Policy Studies and Foreign Policy in Focus, both liberal, anti-war organisations, put the cost of current operations in Iraq at $5.6 billion per month.
This breaks down to almost $186 million a day.
"By comparison, the average cost of US operations in Vietnam over the eight-year war was $5.1 billion per month, adjusting for inflation," it said.
As a proportion of gross domestic product, the Vietnam War was more significant, costing 12% of annual GDP, compared to 2% for the Iraq war.
However, economists said the Iraq war is being financed with deficit spending and may nearly double the projected federal budget deficit over the next 10 years.
The US Congress has approved four spending bills for Iraq so far with funds totaling $204.4 billion and is expected soon to authorise a further $45.3 billion.
"Broken down per person in the United States, the cost so far is $727, making the Iraq War the most expensive military effort in the past 60 years," wrote authors Phyllis Bennis and Erik Leaver.
US public support for the war in
Iraq is dropping
As public support for the war drops, more politicians, including some Republicans, have begun to compare it to Vietnam.
The latest was Nebraska Republican Senator Chuck Hagel, who received two Purple Hearts and other military honours for his service in Vietnam.
He said earlier this month that the US was "locked into a bogged-down problem, not dissimilar to where we were in Vietnam".
The total cost of the Vietnam War in current dollars was around $600 billion and there are some experts who believe the Iraq War will eventually surpass that total.
For instance, the Congressional Budget Office estimated this year that if the United States managed to reduce its troop deployment to Iraq and Afghanistan to 50,000 by 2010, the cost over the next decade would be an additional $393 billion, which when added to the dollars already spent would exceed the Vietnam total.
While there are far fewer troops in Iraq than there were in Vietnam at the height of that conflict, the weapons they use are more expensive and they are paid more.