I believe it’s relative. A fancy dinner for a rich person might be on par with a hamburger at a diner for me, percentage-of-income-wise.
I also tend to think that dining out is often more about the experience, for rich people. That’s what all the music and waiters in ties and fancy drinks are for. I’m sure that the food is important, yeah, but not for filling up on cheaply. If they care about that, they can do it at home.
On the other hand, I see more lower- and middle-class people at places like Chinese Buffet or fast food, where the object is not the atmosphere or dining experience, it’s shoving food (lots of it! supersize!) down the gullet. As usual, when you’re worrying about money, you’re a little lower on Maslow’s Heirarchy and have to worry more about the amount of food than whether or not the napkins are elegant.