You would think it would be chicken. I guess I would hope it was chicken and not something less desirable. With the amount of chicken in commercial food production, I would guess the fat it far cheaper than duck. The main markets for chicken fat are pet food and diesel full. And there is a smaller market as kosher schmaltz, where in small home cook quantities, it's a high as duck fat. In fact, the price of chicken fat as a commodity moves up and down almost exactly with the price of crude oil.
Given the price of duck fat, I would be tempted by a mix that was much cheaper, assuming that the percentage of duck fat was more than some minor part. The percentage of duck fat would have to be substantial, because there is a serious flavor difference, with duck fat being far superior. Nutritionally, they are virtually identical.
Mixing duck fat with another good frying fat, like peanut oil, works well to control cost.