Your claims of "up to $12,000 per month" appear to take the absolute worst case scenario. Wikipedia notes roughly 90% of Xyrem consumers get this via insurance, with co-pays under $50 and a significant number get it for under $25. Admittedly, insurance companies are getting stiffed, but one suspects they're negotiating lower than worst-case pricing with the supplier in the first place.
Being an Orphan drug, development costs are amortized over a very few patients, making costs high. It's like saying a B-2 bomber costs billions of dollars to build when, in fact, a huge cost of "building" the plane is the amortized R&D costs. The actual labor and materials is much less. That's why the fewer planes are built, the more expensive each one becomes.
There is no good solution to this problem. If you fix prices such that R&D costs can't be recouped, you remove incentive for pharma to R&D the drug in the first place. If you allow them to recoup the costs, the end user must pay them. There is no other way. Pharma R&D dollars don't just fall from the sky.