A common mistake beginning mystery shoppers make is that they focus too much on the word "shopper" and just think retail. A typical retail assignment will pay in the $7 to $15 range, although a few can go as high as $25. Personally, I do very few retail assignments because they don't pay as well. If you do higher-paying service category shops (such as apartments, home builders, car dealers, testing centers, gyms, oil changes, banks, brokerages, etc.), you can typically make between $25 and $40 per assignment. A dinner restaurant shop is also worthwhile because it can pay $35-$100 in free food/drinks. The more you are asked to do, the more the assignment pays.
Next - find out what a mystery shopper can realistically earn.