To start with, I had roughly the same factors in my head. Rarity and skill set / experience, especially.
I was thinking of Joseph as worth a lot more than that, because of the sheer breadth of his skill set. No one probably knows just how much he does for Alexi. He's an amazing manager, with everything that entails. And he handles the finances for the Vandros household. I even imagine him doing Alexi's taxes. He's a relatively accomplished musician, repairman, cook, caretaker, gardener, bodyguard etc... because his roles have changed so much over the years. Furthermore, the fact that he's not formally hard broken but just psychologically still in that mindset gives him significantly more ability to act with both grace and independence.
As far as difficulty training him for a new household, I don't see that as particularly difficult. He would go quietly without voicing his displeasure or trepidation in any way.
You've basically covered all of that, so I was still trying to get why he's still not worth much to most people. And I guess what it is is that his skills aren't necessarily slave skills. I imagine that some owners think so little of slaves that they would find Joseph's intelligence and ability disconcerting at best, and downright insulting at worst. Some might be worried that someone with that much independence and skill would betray them, because they are “cut out” for a better life than that of a slave. At some point, being too competent could actually hurt you.
Besides that, I'm sure that Alexi keeps the skills of his slaves, especially Joseph, rather quiet. So no one but Alexi knows just how valuable he is. Joseph is not being graded on his actual usefulness, but rather his perceived ability, in a culture that has certain ideas on what a slave should do, and in a world where it's hidden for his own protection. That's why his score is so low. The people who can see past all of that, Orion for instance, know that he's worth a lot more. So if he were going to be sold, it would be a private thing to someone who wants a trustworthy slave with his skill set. On the open market, against slaves who have only done slave work, his skills don't help him that much.
Of course, it's irrelevant since even Joseph knows that Alexi would never sell him. Tristan came close to it a couple of times though. :-P So, I was kind of insulted on my character's behalf at first, but ultimately I agree with you.
As far as Erik's price... he's got to be almost worthless. He's a vampire, so a common species, with a known violent criminal history. He has some skills in repair work, cooking, stuff he had to do for the Resistance, but he's a major flight risk and would be prone to lash out. He'd be worth slightly more to someone crazy who liked the idea of utterly breaking a formal Resistance leader, but otherwise he's a 2 at best. Maybe a 3 to someone like that.