In order to calculate if the relationship between price and volume (in grams) be described by a constant rate we would need to calculate first the cost of how much 1 gram would be.
For example for the sports drink mix of 150 g of $9
if 150 g is $9. Then the cost per gram is=(9*1)/150=$0.06
We can assume here that a constant rate would be that the price per grams is $0.06.
Therefore, there is a relationship between price and volume in this case, the more grams you consume per product the more it will cost.
