How much can you make mystery shopping? It all depends on what you are willing to put into it, what you are willing to do and what you are trying to accomplish. Can you go on day trips or multi-day routes? Do you need to stay close to home or can you travel a little? Are you looking to have services or meals reimbursed or are you looking for hard cash? Etc.
I teach full time and, without stressing myself too much, can bring in over $1000 in any month I choose. I can do a lot better over the summer.... A lot better.
Hard work builds character and homework is good for your soul.