In business, this would be referred to as the razor and blades model: https://en.m.wikipedia.org/wiki/Razor_and_blades_model
If the sellers of condiments were giving away chicken, or selling it at a deep discount, it would match exactly.
More generally, it’s hard to find a perfect analogy, since there are many things where the value of the basic ingredients is small in relation to the value of the final product, sometimes because of the labor involved, and sometimes because a small ingredient is rare and costly. The right expression depends on the attitude that the speaker has towards this imbalance. After all, the value of a Brancusi sculpture is somewhat greater than the metal or rock that he started with, and most of us are okay with that.
However, there is always the expression: You’re paying for X, where X is something other than the primary ingredient. If the speaker doesn’t think there is value in X, then you’re just paying for X conveys the sentiment.
- You’re just paying for the view (at a rooftop restaurant).
- You’re just paying for the name (a designer handbag).
As I write these, however, I realize that they’re specific to the class of people that can eat at rooftop restaurants and contemplate the purchase of designer handbags. I suspect that the notion of an imbalance in value is dependent on one’s place in society, and that a general analogy will be hard to come by.