Correct. Is that the average price of beef noodles in your city?

What context are you saying this? If you're making a statement, I'd say: "It's 12 yuan for a small bowl of beef noodles."

It's grammatically valid. It's fine to say to a friend, when telling them the price of something they're considering eating. But it's not ideal for marketing, if you represent the shop, because then you would be better using more personal, friendly, enticing and snappy wording.