Well, calculating begins with base factors. In my mind slaves have only a handful of factors: Rarity, Skill Set, Trainability, and Lifespan. 

In Joseph's case, he may be one of the more common breeds, but due to his age his skills are quite varied, and because he has been successfully hard broken for so long, he's also easily trainable. Also, he's immortal, which adds to his value decently, and because of the free reign Alexi has given him over the past several years and his station in the household, he has developed skills far outside the normal range for such a common breed, on in fact, slaves in general. So, on a scale of value to the public, he's only about a 5 out of 10(The work it would take to transition him over to a new family adding extra cost, hence taking away from his value a bit), but for people looking for the exact kind of slave he is, he's valued at at least a 7.

So publicly, he's a so-so slave, but to those who see more value in his talents and grace, he's a fairly decent slave. And because Alexi refuses to even consider giving him up, that just makes people more curious as to why. . . . .

Well, that's my math, what's yours? Oh, and I see less intelligent and/or naturally violent slaves like minotaur and were-beasts as the "Less trainable". Just so you know what I meant by that :P