Skyrim:
Dragonborn-$19.99: actually This DLC is amazing and is worth more than its retail price. It adds a huge explorable island and adds more than 50 full length quests. Worth: $29.99 at minimum
Heathfire-$4.99: fairly decent DLC adds three houses and adopting kids. Not worth the money however buy when it's half off at $2.50 or less
Dawnguard-$19.99: adds 38 new quests if vampires are your thing it's a must have. It's got a few bugs and random vampire attacks killing people they shouldn't but otherwise yes it is worth exactly $19.99
Oblivion:
Knights of the nine: $9.99 amazing several quests in a row and worth $12.99 in my opinion
Horse armor pack: 1.99 it's a joke a bloody joke. I'd pay fifty cents or maybe even a dollar for it. But not 2
Battle thorn castle: $1.89. Why this was cheaper than horse armor the world may never know. It is the only dlc I've never played. But looks to be worth it.
Meh runes razor: $2.99 skeptical on the price but its really not worth if because its only a trophy by the time you get it. Of course if that's all you want or modded you game then yes it's worth exactly $3
Orrery and thieves den: both 1.89 but both should have been included in game in my opinion and this should have been free.
The other three are worth the price. And shivering isles anything less than $49.99 is a steal.