Imagine the blue bag to be 100 units big and cost $100, so 1 unit costs $1.
The red bag must be 120 units big, and costs $150, so 1 unit costs 150/120=5/4=$1.25. The red bag units cost 25c more than the blue bag, which is 25% of $1. That means for the same mass (1 unit) the red bag costs 25% more than the blue bag.