GMAT Sentence Correction (SC) | A new hair-growth drug is being sold for three times the

First off -- make sure you understand the meaning of the sentence. What is the author trying to convey?

In simple language ---> There is a new product. The manufacturer of this new product is selling it for 3 times the price of another product (also produced by the same manufacturer). Both these products contain the same ingredients.

Look at choice D closely --

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