You might consider contacting the insurance regulating agency in your state.
I worked for a company that used Medco and the policy then was that you could get a brand-name medication if there was a generic available, but you would pay the brand-name co-pay AND the difference in actual cost between the generic and brand-name.
However, in this situation, it seems like they don't have a generic of the medication you need, so it might be a different issue. Do they fill it through their mail-order system? Check online to see if they cover it that way at all.
Try going through your benefits department at your work. The insurance companies want to keep their clients (the business who contract with them) happy. If they get in touch with them they might have more bargaining power than an individual or doctor.