Bethesda's suggesting a retail price of £19.99, which covers two major expansion packs, a full-colour map and a bonus disc. No mention of horse armour, though.
Here's the breakdown:
Knights of the Nine: A fallen King has been unchained from the darkness of Oblivion to seek vengeance upon the Gods who banished him. Only a champion pure of heart can vanquish the evil that has been released.
The Shivering Isles: Enter the torn realm of Sheogorath - a world where Mania and Dementia reign. Do you have the strength to survive his trials, to tame a realm fraught with paranoia and despair, and wear the mantle of a God?
Oblivion Game Map: A full-colour map of Cyrodiil and the Shivering Isles
A bonus disc: Featuring a behind-the-scenes documentary on the making of Oblivion