[ghaff]

Lunches tend to be less food than dinner, but that aside, lunches tend to be cheaper than dinner in the US as well--presumably because people tend to be more price sensitive at lunch. I'm not sure I can do a good job of articulating the reasons. It's partly as you say. I think it's also partly because the lunch specials in Chinatown or the sushi lunch are competing against sandwiches and other similar luncheon fare to a degree that they aren't for dinner.