There may be some underlying linear relationship but I have always had a break-point over which the effort and discomfort become significant and quickly get into the miserable to impossible. Sort of like climate change "tipping point". When I carry a heavier pack it is usually for a longer duration trip, like two weeks. Then I do get close to my limit.

Much more complex than a simple formula. The comfort part of it is dependent on your pack fit and suspension and how you pack it. Also lots of walking techniques involved, such as rhythmic breathing or the rest step that allows you to carry heavier loads uphill. Also just plain mental; get in the right state of mind and you can do amazing things. I find unexpected difficulty more difficult than expected difficulty!

In addition to "costs" some weight has "benefits" that outweigh the costs. Not everyone wants to go faster or farther. Depends on your backpack style. Some are willing to "pay for" camp convenience or comfort or the ability to partake in an activity (hobby).