“ Secession would have allowed them to keep more of it, 60% of which was getting grabbed by Washington DC (tariffs) and New York. (Excessive shipping costs, handling costs, insurance costs, warehousing costs, etc.)”
Because of course all those costs go away after secession, and the infrastructure cost of increasing shipping fleets and port facilities would have been negligible./s
The shipping costs would have been massively reduced by the employment of foreign ships to carry the cargoes. This was prohibited in the Union because of the "Navigation Act of 1817.".
Some years ago I had a good series of message exchanges with a man who's family was involved in the Cotton trade and Shipping industry of New York. He confirmed that the whole enterprise was set up to transfer as much money from the South as possible.
If I could remember his name, I could probably go back and find those messages where he explained how the whole system worked. Rustbucket, was it you? If not, do you remember who it was?
As for the dock costs, the warehousing costs, the harbor improvements, and so forth, that would have been money directly improving the economy of the Southern cities where improvements were undertaken.
It would have been money not going to the North, and instead being spent in the South. If you read about the boom in Charleston after secession was declared, you could understand that it was going to be massively profitable to them.