I'm not a sociologist, but I've had a longstanding interest in the origin of manners, so I've read a bit on this. The problem here is that putting a number on something invites comparisons. We almost can't help comparing numbers. If my friend gets a number and I get a "You are awesome! Thank you so much!" I can't easily compare those because they are unlike each other. But when my friend gets a 500 and I get a 12.5, it is obvious to everyone that the number I got is so much lower, it's embarrassing. That guy's number is way bigger than mine.

Basically, no monetary reward is necessarily expected here. But once you give one, it is a faux pas to give one that looks small in comparison to what is considered the "norm." We even have common insults reflecting this fact, where bragging about something I impressive will invite comments like, "Wow, here is a shiny nickel." The impromptu offer of a clearly paltry reward is understood as a slight.