cheap dinner near third and market in Phila
the pagoda noodle house! it's just a couple blocks away. walk south on 2nd, and just after chestnut / before walnut there is a left turn into the sansom walkway with the ritz east movie theatre, it's in there. awesome, well-prepared CHEAP asian food. my staples are the indonesian tofu soup, singapore noodles, fried bananas.
also, i usually just drink at the society hill hotel (3rd and chestnut, i think?) - it's a really great bar, especially compared with the area's other offerings. but this last time i was famished and had a tomato bisque which was tasty and rich, perfect on that cold cold night.
eulogy is great as mentioned, as is la locanda del ghiottone. i think that about lists my favorites within a short walk.
Depending on the type of food you are looking for you have quite a few options:
Rotten Ralph's at the corner of 2nd and Chestnut is decent bar food, as is Nick's Roast Beef on 2nd.
On Chestnut you will also find Kabul, a nice Afghani restaurant.
If Sushi is your choice, Zento Contemporary is also on Chestnut
Triumph Brewery and Eulogy are also right there on Chestnut.
All of these places you can get out of there for under 30 a person, And a lot less at several of them.