I think the price can vary based on a lot of factors. The breeding pair you mentioned is not a "matching" pair so, in my mind, they aren't worth as much. I have seen a mating pair go anywhere from $200 up to $1,000. It really depends on what it is worth to you. I think that the bottom dollar for fish like that is probably around $200-250.
As for keeping multiple breeding pairs in the same tank, all the research I have done (I don't have any yet so no actual experience) says that the breeding pair should be separated from other fish. It is actually better to keep them in a slightly smaller tank (I see 30 gallons recommended a lot) so that the fry can easily find their parents, since they start off feeding from their parents' slimecoats. If you keep any pair in with other fish, even other discus then you run the risk of either the other fish eating the eggs or fry, or the parents getting spooked and eating their own eggs or fry. Even in their own tank the parents can get spooked and eat their own eggs or fry.
