The problem with the cost of the DLC is that it really hasn't got much to do with customer perception. Microsoft and Sony will demand a certain price in most cases, making your plans completely random. That's because when speaking DLC with them, all they care about is the weight of your additional content in Gb, and not the number of hours it includes. Brand power is a factor in as well of course
As far as your question goes, I'd say that the best approach may be to quantify it in percentage terms of the total length of the original game, and again the nature of the DLC?
Precisely for an RPG, typically long games, a 30 hours game that gets a 2 hours extension for 10 bucks may not be worth it (1/15th extra game length for 1/6th the original price tag). But a 5 hours extension for 5 dollars is a lot more bang for your buck (1/6th extra game length for 1/12th the original price tag)
Again, depends on the nature of the DLC. If it's extra content that radically changes the entire experience and is accessible at any time (think Joker specific levels in Batman, that act as a mini prequel to the game), then every fan will want to experience it and you can charge a little extra. If your DLC typically caters to a very small niche of your entire game market, pricing should change to reflect that.