In many kinds of research, I didn't have any budget or money to pay customers, and in educational courses on customer research, it was strongly advised not to do so. For a while, I thought it was the market standard — not to pay — but it is not.
I believe that money exchange between a researcher and a respondent enables the same trigger that switches whenever you ask a respondent a hypothetical question. The respondent will tell you anything you want to hear if he has money leverage at the end of the road.
This is coming from my experience, I've heard many respondents asking me "can I talk like that about X product?" or "will you pay me after I've said that?". The fear of losing money for talking negatively about the product is very common for products from 20-30k MAU when you're becoming massive rather than being exclusive to friends and family.
This is basically what is called The Hawthorne Effect when individuals change their behavior in response to the outside factor.
