According to the 1860 census there were 3.95 million slaves in 1860. If the government had paid each slave owner $800 per freed slave it would have cost a little over $3 billion to free them, compared to over $5 billion to fight the war on the North's side, which doesn't count the South's cost, the cost of years of disruption to the economy, the loss of life on both sides, the years of disruption to the economy from lost lives and from injured veterans being less productive, and the cost to the economy of the war and the ironically named "reconstruction" leaving the South an economic wasteland for nearly a century.

Freeing the slaves and paying slave owners fair market value in compensation would have cost a tiny fraction of what fighting the war eventually cost. But the Civil War wasn't just about slavery. It was also about the rights of states to make this determination for themselves, and not have it mandated by the Federal government. It was also about the rights of new territories to make their own determination on achieving statehood through local elections. And it was about the forgotten part of the cause of war: concerns and/or fears on the part of poor white southerners about what would happen to society if nearly 4 million slaves were dumped from plantations out into the world. None of that goes away just because the Federal government decides to spend over $3 billion of money it doesn't have to buy the freedom of the slaves through a mandate that would have been clearly unconstitutional in 1860.