Shivering Isels ran for 30 bucks... and once i got it, i pretty much stayed there. If you were hell bent on it you could pound out the main quest line in ten hours. A waist in my opinion but you could do it none the less.
Dawnguard's MQ is stated by the dev as taking ten to twenty hours if you rush it. With oodles of side quests, things to fight and places to explore. I personaly expect to spend many an hour just finding enough bandits to eat to lvl up my werewolf.

and for us xbox owners it'll run 1600 micrsoft points. about twenty dollars US.
playing with only 1 character and only doing the MQ that runs ya about a buck an hour for new content and gameplay on one of our favorite games. I spent more on games in the arcade growing up and i didn't even get to keep them after.