To build upon my earlier point, divide the quest into stages and create a minimum bid based on how many stages have been completed?
A random example:
Collect X water, Y herbs, Z branches is stage 1 --- complete this for a minimum bid of 10 silver
Collect 2X water, 2Y herbs, 2Z branches is stage 2 --- complete this for a minimum bid of 20 silver
and so on.
But yeah, this kind of looks too demotivating (both for the slave as well as the buyer, who may not want to pay much). Scratch this part then lol.
Edit
but then again, the real question is: to what extent is a wanna-be slave willing to work to get his leash?