Completely unknowable as framed
You have to factor in too many unknowns.
What's the expected long term interest rate? 1%? 8%? 15% That makes a huge difference to the real cost of a slave, just like for a house. Price factor: x100+x100? Certainly x10.
What's the perceived political risk relating to forced emancipation? If you think your slave will get set free by politicians in 2 years time, you simply won't pay for 40 years work. Price factor: x100
Who is the slave? Is it a gorgeous 22 year old woman with great management and conversational skills, and a submissive personality? Or is it a 60 year old unemployed chain smoking heroin junkie with a record of violent crime? Price factor: x100+.