- Doing 1 hour of effort to save 5% on your $20 lunch is foolhardy for most people. $1/hr is well below US minimum wage. - Doing 1 hour of effort to save 5% on your $50k car is wise. $2500/hr is well above what most people are making at work.
It's not about whether the $2500 affects my ability to buy the car. It's about whether the time it takes me to save that 5% ends up being worthwhile to me given the actual amount saved.
The question is really "given the person-hours it takes to apply the savings, and the real value of the savings, is the savings worth the person-hours spent?"
Also, this is exactly what you company is doing, paying you to "read the book" so they don't have to.
I'm sure you can use your imagination to substitute "lunch" and "car" with other examples where the absolute change makes a difference despite the percent change being the same.
Even taking it literally... The 5% might not tip the scale of whether or not I can purchase the car, but I'll spend a few hours of my time comparing prices at different dealers to save $2500. Most people would consider it dumb if you didn't shop around when making a large purchase.
On the other hand, I'm not going to spend a few hours of my time at lunch so that I can save an extra $1 on a meal.
If that moves the needle on your ability to purchase the car, you probably shouldn't be buying it.
5% is 5%.