The idea of Lincoln buying the slaves instead of going on with the Civil War comes from the book “The Peculiar Institution” by Kenneth M. Stamp. Mr. Stamp also gives us the economic costs in dollars and shows us that that it would have been financially less expensive. Some slaves were able to buy their freedom. Lincoln was part of an organization that was sending freed slaves to Liberia which was a U.S. colony at the time.
And what reason would the southern slave owners have had to sell? What would they replace the slaves with?
Sure buying the slaves, if feasible, would have been cheaper than the war. You can say that about any war. But the wars come.
Given that the average price of a slave in 1860 was $800 and that there were 4 million of them, that's $3.2 billion. The entire federal budget in those days ran about $80 million a year.