So... yeah. Let's not get too attached to in-game prices.
Sage advice.
Anyway, purchase price of a slave wouldn't be useful for determining cost, because slaves had to be provided with food, shelter etc. in order to work, too.
The debate about whether slave labor was, in fact, relatively cheaper than free labor is at least two hundred years old; it was one of the first questions that economic historians ever considered, and they still haven't solved it.
On one level, the discussion is arguably irrelevant, because basic productivity models have shown that cheaper labor is cheaper precisely because it is less productive. Considering the relationship between labor costs and price without factoring in the different levels of production by various laborers is pointless. Now, there've been some historians who claimed that slave labor was in fact quite an expense for many owners, therefore justifying the claim of high productivity. And others have argued persuasively against it, and have also pointed out that owning a slave was not a purely financial calculation - there were elements of prestige and social worth, along with psychological power, to be derived in addition to the actual fruits of a slave's labor.
There's also the wild card of the labor source in and of itself. Many Tevinter imperial slaves were formerly free citizens of the Empire, but many were foreign slaves that would not be available to the Empire as free labor. Perhaps per-slave productivity is indeed worse and cheaper, but because the Empire can draw on a pool of slaves it would not otherwise have access to, it can still produce
more than free competitors.
Feh. Point is, if you follow this all the way down you run into lore problems - as exist with any lore for any fictional setting ever, because expecting authors or game devs to be historians and economists and scientists with maximum competency in their fields is stupid - and you run into internal contradictions, and you can explain a variety of opposite things equally well (and equally poorly). Attempting to divine hidden facts behind the game lore is less "thought experiment" and more "intellectual self-pleasure".
As a side note, the Spartans had a similar tradition whereby a slave was released and boys, in order to become full warriors, ordered to track them down and murder them. This is, of course, despicable, but the Spartans were hardly a cowardly people.
We've mentioned this a couple times in the thread. The
krypteia wasn't like you describe - the slaves weren't "released" (they lived in Messenia, which was already their home, because the whole country was enslaved; where would they have gone?) and most Spartans didn't participate, such that it was more like an "elite" unit of slave murderers rather than a rite of passage for "boys to become full warriors".
I don't know that I would describe the Lakedaimonians as "cowardly", because I feel that applying personal attributes and traits to an entire society is stupid. But the practice of murdering slaves, with state sanction, is in my mind unquestionably a cowardly act.