I’ve nearly given up on this one. But in the spirit of spitting into the wind, here goes:
Both words refer to smaller amounts of something. If you can count the individual things you’re describing, you say “fewer.” If you can’t count the things, you say “less.”
For example, we say, “I put less salt on now” because we aren’t into getting an exact count of salt crystals. We also say “less water” and “less time” and “less pain.”
But if you can count the things you’re talking about, or they each have individual worth even if there are many, we say “fewer”– such as “fewer people” and “fewer houses” or “fewer incidents.”
We can’t accurately say “We have five less cars on the lot” because we hope that we can count those cars. We say “We have five fewer cars on the lot.”
You could accurately say that something costs less money and be right. You could also say that it costs fewer dollars, and that would be correct. But it would be weird.