I assume he knew the price differential. The price differential today is less than when he wrote the teshuvah, which was before Goldman’s started competing.
So, although he didn’t give specifics, it’s safe to say that it would certainly apply today as a general rule, at least in the NY/NJ area.
Of course, some individuals may be in such a financial state that they are justified in buying the cheaper c.s. even according to R’ Moshe.