Thanks for the input. This has given me lots to think about.
The kingdom I'm using for this is called Calastia, part of the larger Hegemony of Calastia. In the fantasy setting of Scarred Lands, and their values seem to be something in between what has been discussed above.
Slaves are commodity, but they cannot be sold / bought / gathered just anywhere. There are actually only two countries (although fairly large in comparison to the other one's) on the whole continent, which deal in slaves. Dunahnae and Hegemony of Calastia.
Now, if the characters were in Dunahnae then the reality would be that a slave isn't worth the spit on your boot. The whole country is basically run by a very small nobility class, almost a non-existent merchant class, a huge military and then a hoard of slaves. Slaves are bred for particular qualities and then worked until they die. Preferrably a "good slave" would be used for reproduction at some point, so the "good qualities" would be passed on to the next generation of slaves. Other than that...killing a slave means nothing there...The military of Dunahnae constantly raids for slaves from near-by communities and breeds its own to cover the large death rate. The nobility are haughty and uncaring for anyone except themselves. The quality of slaves is very low in Dunahnae, but they are in great quantity.
Thankfully the characters are Calastian nobility, and reason and pragmatism comes first in that country. While slavery is accepted it is actually practiced on the larger scale only in two of six duchies. The main reason, that I can see, is that controlling slaves isn't very practical when one can hire readily available men and women for just about any given job. Trained, highly trained or otherwise.
The two duchies that make the exception are Turrows and Jandalore.
Turrows is a militant duchy, mainly because it has the finest military training facilities in the world (or so they claim) and is also the backbone of the kingdom's military. Large numbers of mercenaries also gather here, along with all manner of other people. Including merchants, slavers, assassins and so on...On the outside Turrows seems quite a lot like Dunahnae, except for the more pragmatic set of mind that Calastian's share at large. While slaves have no rights they are treated well and kept alive as long as they will live. Reaching a venerable age usually just means that the slave will be posted to less physical labor. Retirement comes with death. While Turrows has nowhere as many slaves as Dunahnae they do seem to be of higher quality and of longer use to their owners.
Jandalore in turn is based on its maritime exploits. They have the greatest navy and largest maritime merchant trade in the world (or so they claim). And they ship in slaves from all over the world. Thus the quality of slaves is a bit lower than in Turrows, where quality comes before quantity and slaves have to be brought in over land. But again the pragmatic nature of Calastia sets in, and no slave is really treated harshly once they reach Calastian soil. There have been no slave revolts in the history of Calastia, because the slaves are too few, too far apart and treated in manner, which doesn't leave much to complain about. Certainly they are not humans (or whatever their original race) anymore, but that doesn't mean that a tool should be mishandled or thrown away after use. That just isn't practical.
The one major difference is that any common man of Dunahnae can be capture by a noble and made into a slave. In Calastia no free man (and all men and women are born free in this land) can be made a slave, unless he has broken the law in a serious manner. For those cases there are specific rules. This presents a small problem to the slavers running their trade there: they have to fetch their merchandice from outside of Calastia, and since the same laws abide across the Hegemony of Calastia that is a whole lot of land to cover. The Hegemony of Calastia is about 1000 - 1500 miles in length and latitude, after all. Of course sometimes the occasional peasant, laborer or expert might disappear, but this is more of a anomaly than a rule.
Noting that those born on Calastian soil are free men and women there is also the problem of not being able to breed slaves within the Calastian Hegemony's borders. Of course there are ways to go around this, but the general mood of the nation is that a child born here is no one's property. It doesn't say so out right in the resource book, but that's my interpretation of the letter of the law.
So, slavers have to travel thousands of miles to gather up the various slaves, and to conduct armed raids into suitable locations. They also have to find trained, healthy and young slaves, if they are to make good money. For Dunahnae anything that moves, has a suitable number of limbs and breathes (the last one isn't really a problem either since they practice necromancy heavily) is good enough. For Calastia I would view slaves as a more of a nobleman's mark of pestige. Common laborers, craft masters and such become much cheaper and are much more readily available. The kingdom of Calastia alone has a huge population, which is for the most part civilized and well trained in their various crafts and areas of expertise.
Although the owner of the slave can basically do what he wants the King frowns heavily on abuse and poor management of slaves, because it incites rebellion. And there have been no rebellions in history of Calastia. Period. (Or so they would have the common man believe)
This leaves slavery as a second rate commerce at best. Still, slavery does exist in two duchies, so there are enough decadent lords out there who want their own pets, laborers and professionals for life. Dunahnae doesn't pay more than a few copper / silver pieces for a slave. Calastia pays better, but also expects far better quality. This leaves your average slaver in the middle with a problem of pricing the slaves correctly.
Prices in the tens of thousands seem ridiculously high. A hundred pieces of gold for a slave that can be worked for 50 years seems rather low as well. Especially if that slave has already received training, and can be easily trained further by the new owner. So, I guess that the correct answer is somewhere in between.
Any suggestions?