Secret shoppers may earn a 'high' hourly pay, but remember that you only get paid while you are 'shopping'.
Example: you secret shop a fast food restaurant. You might earn $15 an hour, but only get paid for the 30 minutes you are actually there eating. The pay does not include the time you spend traveling to and from the restaurant, or writing up the paperwork evaluation and submitting it. You often have to pay for the meal yourself (and order what THEY want you to order), then submit the reciepts for reimbursement.
I have friends who tried it, and found that the number of assignments was too low (you can't shop the same place too often, or you are recognized), that they had to travel too far to get to the assignment, and that the total hours paid was too small and too irregular to make it worth the effort. The organizations that pay secret shoppers rely on lots of new applicants, who are assigned a few quick jobs the first weeks and then rarely get called again. Missing or turning down even one assignment can keep you from ever getting another assignment.
The ones who get frequent assignments are those who provide a well written, complete evaluation. Good grammar, good use of descriptive words, complete sentences all make your report stand out from the others, and make you a more valuable employee.
Don't count on even one assignment a day, even if you are the best shopper they ever had.
Oh, and you probably won't get assigned to stores and restaurants in your immediate area -- they won't want your judgement clouded by past experiences there, or the possibility that you or a friend/family member works there.