The hubs have some value, because they haven't been made in decades. If the rotors are decent, that adds a bit to their value. While new Chinese rotors are available, they're not the quality of those from back in the day. However, if your rotors are no good, they actually detract from the value of the hubs, because someone has to switch them out, and pay to have them shipped, only to junk them.
If by backing plates you mean splash shields, they are worth something, too, if they are nice, or could be made nice. If you mean supports for front drum brakes, I'd think they have minimal value.
Rebuilt calipers are readily available at $100, so depending on condition, from rebuildable core to really nice, you could price yours accordingly.
Your master cylinder, booster and proportioning valve can all be used on other disc systems.
I don't have prices to suggest for you. Looking through the old for sale thread should give you plenty of guidance.
