There really is no "average", the numbers will run the gambit, but I would look at 70-30 beverage vs. food, or a higher percentage on the coffee. We're roughly 76-24.
You also want to not at profit margin, waste, and the ever important, "What the hell are you?" Design your business model so you can survive without food. If you base your model on food as a necessity rather than an additional benefit, your business life will be short.