This is a teachable moment.
The value-added of a business is not relative to the price of the materials used in the manufacturing.
The value added is a function of the utility that you derive from the consumption of the materials post-processing.
Without arbitrage no one is willing to pay for the price of materials and labor; they pay for the utility that they derive from the finished product.
Someone at McD has determined that there is $1 of utility attached to a slice of cheese. They have also determined that, on average, there will be 50c of utility derived from the sauce.
They probably found it was like 102c and 53c but rounded it off because of the dis-utility of un-even numbers when adding things on.
The point being that they are there to make the most money they can; they are, therefore, gaming your gaming of their system; while still allowing those that find value in add-ons to do so.