So to keep the money flowing they destroyed the south’s infrastructure and freed their unpaid labor. Seems like a funny way to make money.
They initially sought to keep the money flowing. When the Washington power initiated the war, they all thought it would be over quickly. When they eventually realized it was going to be a long slow horrible trudge, it became in their best interest to destroy as much of the South's economic power as possible.
But to clarify their motives more, it wasn't just about the South producing all that money. It was a far bigger threat than just losing that money stream. The South was going to allow much cheaper priced European goods to flow into all the states with access to the Mississippi river watershed, and this would wreck the domestic markets of the big money power industrialists in the North East.
It would also start the Union hemorrhaging states, territories, and economic power. No, the Independence of the Southern states was a grave threat to the economic interests of powerful people who pretty much controlled the US Government.