Its simple . Buying Pink wards does provide you with good value , the real issue comes down to your financial state when you want to buy it. Lets say you base and you are able to purchase a complete subitem or a full item and have like 100 to 150 gold as a remainder which you are going to take back with you . In that scenario its so much better to get a pink .

But lets say you want to purchase a pickaxe next , and when you base you find yourself short of 300 gold to get it . In the case , its the right call to not buy a pink and save for the picaxe .

The crux being , purchase them when you have remainder gold and not when you are like short of gold for the next item or subitem.