Here the scale of time is larger and does make the 5$ significant, while it isn't at the scale of a few days.
If that moves the needle on your ability to purchase the car, you probably shouldn't be buying it.
5% is 5%.
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.
- 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.