The Elder Scrolls IV: Shivering Isles Release Details

Kotaku is reporting that Bethesda's Shivering Isles expansion for The Elder Scrolls IV: Oblivion will be released on March 27th and will cost $30 at retail or 2400 Gamer Points on Xbox Live. Shouldn't a downloadable version be cheaper since it costs the developer less money to get into gamers' hands?