Late comment on this,

A year or two ago I found a website for mixed raced Americans (Black/White Americans). One link was to the history of the Abolitionist movement and its growth. The author made a historical point of the fear of white slaves was the biggest motivator in the movement. They had a point. Every decade the appearance of more and more white than black slaves was evident. With no new slaves from Africa coming into the United States in real numbers it was a mathmatical and biological certainty that the slave population would be more white than black in a few generations time.

The market for true black slaves drove the prices up and for mixed raced slaves the prices went down. The fact that mulatoos and quadroons could also escape into the populations easier and were harder for slaveowners to reclaim had a big part in the market prices of slaves too.

David