It's all right but unimaginative. You can be more creative: 'Gongbao chicken for 20 yuan ! Can you afford not to ?'

Yes it's fine. But most static ads would say "gongbao chicken" and "20 yuan" separately in a different style of writing to each other, to keep things very simple and easy to understand at a glance without really needing to read any sentence.

The ‘for’ sounds weak and the same is true for ‘Would you like …?’ Consider just: We have chicken! 20 yuan or Chicken 20 yuan I’d worry more that the ‘gongbao’ description is clear. There can be lots of different anglicized versions of Chinese words so you might want to verify that what you are writing is clear. (Is this what is usually called ‘Kung Pao’ where I live?) Lots of menus show pictures.