But if Deus Ex is such good value, why not just go buy another copy of it then, and get another 27 hours out if it? You can't. You can't get that experience back in the same way because you've already had it. So in some ways it doesn't make sense to compare the two. Were the DLC stand-alone, I hadn't played either that or the main game, and I was buying just one, then obviously the main game is much better value.
But that's not what is happening here. You can go buy another full-price game or an indie title and you'll get much better value. And if value is all that matters to you then DLC should never even enter your mind. It'll always be a rip-off. In fact you should probably be purely playing JRPGs and MMOs. But the question is not "would you like some entertainment?" it is "would you like a bit more of a game you really really like? "
If the answer is yes, then the publisher can choose what they want to offer you it for. Likewise you can say "nah, not worth that much" as I'm sure many people will do (including me, most likely) - we're not the target audience.