How is Dawnguard worth only $10 when KotN was $10 5 years ago? Dawnguard has much more new content than KotN had, just look at these links and compare...
New quests
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_Quests
http://uesp.net/wiki/Skyrim:Dawnguard_Quests
New places
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_Places
http://uesp.net/wiki/Skyrim:Dawnguard_Places
New items
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_New_Items
http://uesp.net/wiki/Skyrim:Dawnguard_Items
New Spells and Effects
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_Magic
http://uesp.net/wiki/Skyrim:Dawnguard_Magic
New NPCs
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_NPCs
http://uesp.net/wiki/Skyrim:Dawnguard_NPCs
New Creatures
http://uesp.net/wiki/Oblivion:Knights_of_the_Nine_Creatures
http://uesp.net/wiki/Skyrim:Dawnguard_Creatures
If anyone thinks that KotN and Dawnguard have the same value, I would like to know your resoning.