What would it have done to an economy that depended on cotton?
Probably not a lot: the subsequent share-cropper contracts were a less effective arrangement than if the blacks had owned their lands themselves and had the planters had to "get competitive" from the get-go. They still ended up relying overwhelmingly on cotton.
I think you mean "What would it have done to an economy almost exclusively dependent on agriculture?" Since slavery was used not only for cotton but also tobacco and rice, and to some extent indigo. Of these tobacco was considered the least onerous re: slaves, while rice was far and away considered the most brutal.