answer: report from congress in 1863 … the cost of fighting the war 2.5 million a day
Source Civil War Quiz and Factbook by Rod Gragg
Edit - The answer to the question depends on the time period you want to look at - the beginning of the war, the end of the war, the midpoint of the war, all four years put together, or all four years put together plus veterans' benefits paid long after the war was over. This was not specified in the question.
I believe everybody's answer was supported by one source or another, so everyone who responded will get credit for a correct answer.
hoosier