MP3 I finished in 10 hours for $60 MSRP.
Spiderman I heard can be beaten in 6-8 hours and fully completed slowly in 15 hours.
Dragon's Dogma is one of the longer ones at 20-25 hours, but can spend 50+ hours doing side quests etc.
Mass Effect 3 (not really new) is 30ish hours.
All the above games retail at $60 when released. Granted they were "new" gameplay rather than an extension of all ready made gameplay, but hour wise I think DG is priced right.