low price
"KoK page 120
Assuming an adult unskilled slave. the cost is 10gp.
laborer is 25gp/npc level"
Entirely too cheap. Granted, you might manage such prices right after an entire enemy army has been captured and enslaved, but normal prices should be much higher.
As a low casual figure, recall that slaves were selling for $2000 in the US just before the Civil War, when $1 a day was good money. Given DMG p 149 says a mere porter gets 1 sp a day, that makes our base price at least 200 gp, subject to major markup for skills, etc.
Note too our PC starts out with 100 gp or more. Saying he could buy an small army with his coin just won't do.
By the way, there is, for obvious reasons, a direct relationship between how well slaves are treated and their cost. The low cost society regards them as disposable items, who can be casually worked to death. Given the hesitation to allow this at all, the DM will likely be more comfortable with a high price that makes the player hesitate before buying anybody.