I assume Blu-Ray discs are still more expensive than DVD discs, and if the features are all in SD and can fit on a regular DVD, that's probably the preferable option to the studios. However, as per the Blu-Ray.com FAQ:
According to the Blu-ray Disc Association, the overall cost of manufacturing Blu-ray Disc media will in the end be no more expensive than producing a DVD. The reduced injection molding costs (one molding machine instead of two, no birefringence problems) offset the additional cost of applying the cover layer and low cost hard-coat, while the techniques used for applying the recording layer remain the same. As production volumes increase the production costs should fall and eventually be comparable to DVDs.
Then again, the Blu-Ray Disc Association is an industry consortium with the sole purpose of promoting the format, so of course they'd claim their format isn't more expensive.