Slavery exists in many nations in Trollslayers. The reasons, and justifications, may vary, but for simplicity's sake the economics are fairly straightforward.
Slaves fall into roughly four classes: Children (usually too young to have any notable Talents, and will almost never have a Profession), General Labor (working the salt mines, housework, etc), Pleasure Slaves (such as dancers or concubines), and Specialists (gladiators, scribes, etc).
The cost of a slave is mainly dependent upon what primary Attribute is considered most relevant to the tasks the slave must perform. Common examples would include Knowledge for a house scribe, Presence for a concubine, Dexterity for a gladiator, and either Strength or Endurance for a laborer (depending on the type of work called upon).
Some attributes act as a modifier to the primary attribute, if high enough to have an attribute modifier, and also relevant to the slave's primary function. For example, while Dexterity would be the primary attribute considered for a potential gladiator, Strength, Endurance, and even Presence could also modify the price thereof. The attribute modifiers are added (or subtracted, if the attribute is low enough, although Children are usually exempt from this unless their attributes are really low) to the primary Attribute for purposes of the slave's price.
If a character's Profession is relevant to their desired slave-task, add +2 to the score. For each +1 in a relevant Talent, add that to the score as well.
That total score is the base cost in Gold Pieces, with the following multipliers:
General Labor x2
Pleasure Slaves x3
Example: Groff the barbarian hasbeen captured by slavers, and is being sold to a gladiator school in a nearby decadent nation. He has a STR of 14, a DEX of 15, an END of 17, and a PRE of 12. His base cost is 15; a +1 is added for his STR, and a +2 for his END. There is no modifier for his rather average PRE. Being a Warrior, with a Profession of Barbarian, add a +2, and his Talents of Sword +2, Shield +1 and Dodge +1 adds another +4, bringing his sub-total to 24. As he is being sold as a specialist, that number is multiplied by four - Groff is sold for a whopping 96 GP.
Another example: Taara is a scribe and translator for a wealthy merchant. She has a KNO of 16, and gets a +1 for her PRE of 13 (as she has to deal with others frequently as a translator). She has a Profession of Scribe (+2), and gets another +3 for the three extra languages she knows. Should her master decide to sell her, she would be worth 88 GP.
If the character has a notable defect, this will reduce the overall price by -25 to -50%. What counts as a defect may vary depending on the type of slave desired: multiple body scars would be considered a defect for a dancer or a concubine, but would probably not affect the price of a manual laborer (and might even be considered an asset for a potential gladiator!).
If slavery is illegal in a given area, the cost for purchasing a slave is usually tripled.