I'm a "secret shopper" for a major big box chain.
Baseline pay is $15/month for minimal participation, like adding to online discussions or answering surveys. Occasionally there are special projects with extra pay. For instance, recently I had to go to my local store and take pictures of a particular area, then upload them with my opinions of the layout.
The most I've earned in a month was $310, right before Xmas last year. Of course, the most opportunities are for Xmas shopping season. ;)
But for the down side, I don't think you can apply. I received an email asking me to join the community. I do not know how or why I was chosen.