A new hair-growing drug is being sold for three times the price, per milligram, as the drug’s maker charges for another product with the same active ingredient.

at which

我觉得应该选C，that作price的定语，就是the price that the manufacturer charges for another product.

price of what感觉有些不对头。请大家指点！感谢！

如你所说，那么the price在从句中做什么呢？

倍数+the+noun+of －倍数表达法之一。