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.
Since the first product sold for three times the price, the drug maker charges another product in the same price.
So, we can use "so" or "as" in the dependent clause.
B. "Than" is to compare one with another. The author is not comparing anything here. He/she is relating two products.
C. "that" - Same thing to compare one with another.
D. and E. "of what" and "at which" - It gave me an awkward sentence.
Hope this will help you. thx.