According to the above letter quoted from Rav Moshe, it is not proper to rely on the leniency of chalav stam if one can get Ch”Y milk with a little more effort and price.
So then, how do you define effort (Tircha) and price. Usually 1/6 is the amount for overcharge. In this case, the difference is usually more than 1/6. More like %50 to %100 (or more if OOT).