For small amounts, the "lore" is that people response especially well to food. That is, more people will fill out a survey for a Snicker's Bar than if offered the equivalent amount of cash (e.g., $2).
Yeah cause I normally won't buy a Snickers it's not a normal thought except when it's in front of my face. I assume that's the case for most everybody.
I've been paid for a questionnaire on a new beer over 40 years ago and another more general one.