Well, it all depends on the expectations and perspective. I've seen shoppers that make over a dozen or two dozen thousands of dollars annually just from shopping, doing all kinds of shops on a regular basis, but also some that only do a shop or two every once in a while, and spend a lot of time doing it.
I like to think of secret shopping as a "buffet" - you will get as much as you are willing to reach out and take. And iSecretShop is a large one at that - there are over 20 different MSPs posting their shops regularly.
I hope you both find something worh your time, and if there's anything we can do to help, feel free to let us know!