Emerson's journal, 1857:
In the south a slave is bluntly but accurately valued at 500 to 1000 dollars, if a good working field hand; if a mechanic, as carpenter or smith, at 10, 15 or 20 hundred.
A Mulatto girl, if beautiful, rises at once to high estimation. If beautiful & sprightly-witted, one who is a joy when present, a perpetual entertainment to the eye, &, when absent, a happy remembrance, $2500 and upwards of our money.
This was brought to mind by Peter Frost's compilation of references on the White Slave Trade (Europe to North Africa) which was of broadly the same size as the Black Slave Trade - involving about 1.5 millions of people.
The White Slave Trade continued on a large scale until nearly 1800, and on a smaller scale for another hundred years - in other words, again, about the same duration as the Black Slave Trade.
But the White Slave Trade was mostly of women, for sexual purposes.
The list of slave prices suggests that, by the time it ended, sex may have become the primary motivation for slavery in the United States - slavery existing mainly for sexual rather than economic benefits. At least, that is what Emerson apparently believed.