You can clearly see the cost of modularity if you compare my current laptop to my previous one.

My current laptop is a 15" retina MacBook Pro. It isn't upgradable in any real sense of the word.

My previous laptop was a 15" MacBook Pro. I could upgrade the RAM and HDD, and with little difficulty replace the battery.

The upgradable one is 5mm-ish thicker and about 400 grams heavier.

Not a true apples-to-apples comparison, but it does show the bulk added by having end user interchangeable connections on the hardware.