For 30MB -> 20MB, CPU is more expensive than Space.
For 30GB -> 20GB, Space is more expensive.
For 30TB -> 20TB, Space is far more......
I do not understang your math.
Cost of sum should be equal to sum of costs
The CPU's verification need be done only once, but the storage occupation are forever.
I have two 1TB HDD in my computer.
First one is 90% full, second one is 10% full.
Does it mean that there is difference in their cost?