DVDs can be produced at a constant marginal cost, and Roaring Lion Studios is releasing the DVDs for its last two major films. The DVD for Rambeau 17 is priced at $20 per disk, and the DVD for Schreck 10 is priced at $30 per disk. If the Lerner indices for Rambeau 17 divided by the Lerner index for Schreck 10 equals 0.5, what is the constant marginal cost of producing both DVDs?