How did the slave traders make such a profit?

Slaves cost about $2,000 each in the late 1700's. With 33.33% inflation they cost about $66,000 each. From here on out all calculations are in modern currency values:

Slave ships took 4 months each way and 2 months to sell their load of about 400 slaves. Also there was a 40% mortality rate so there was about 240 slaves arriving to the United States.

240 x $66,000 = $15,840,000

Then it cost about $200,000 to feed the slaves, $350,000 to pay the crew, and about $50,000 maitanence on the boat.

Therefore the profit would be $15,240,000.

Then the auctioning companies who sold the slaves took about 30% of the profit leaving the slavers with $10,668,000 profit.

This leaves them with time to make two trips a year with a grand total of about $21,336,000 a year.