Specifically, macroeconomically and microeconomically. All sources seem to indicate that it was very profitable microeconomically (for the individual slave owners), so my question is why didn't the cost of slaves increase if it was as profitable as it is. Presumably if it was so profitable for slaveowners plantation owners and others would be eager to purchase slaves and drive up their price to the point where some equilibrium in the market was reached.

Did this happen, or are most sources right that it was very profitable for slave owners and why? General commentary on the economics of slavery is also appreciated.