You buy a material that’s say 5 sq. m and you will cut out something that’s only 4 sq. m from the original material. How much will the 4 sq. m cost for you?

Mathematically it would cost less since you’ve got less material, namely 25% less. If you had bought 4 sq. m materials, you would only have paid for the amount you actually used.

However, if you buy 5 and only use 4, your expense is for the 5 you need to account. You bought 5 sq. m materials to get 4 sq. m. Your expense hence is for the full 5 to get only 4 sq. m. 

The reason is simple, although it could also be 25% less worth if you count the price per material it gives, you however paid the full amount to get less material out of it. You created something so you gave additional value to the material. As such, your expense for 1 sq. m is higher than it was originally and it’s all about you creating value.

Your expense per 1 measuring unit is always what you paid for the material you bought and used for creating the 1 unit.