Tulcingo Del Valle [665 Tenth Avenue, New York; Menu]
Very cheap and very authentic Mexican food is on offer at Tulcingo Del Valle (and at a number of other restaurants on 10th avenue apparently). Oh, and if that's not enough for you, the portions are huge! I ordered 3 soft tacos (1 fried pork, 1 grilled beef, 1 grilled chicken) and I could barely finish my meal. Throw in a Jarritos soda and you'll feel like you're in Mexico, although the food is simply good, not spectacular.