Salary
Average Salary: $14,000
Expected Lifetime Earnings: $584,472
Your best chance at making anything close to a living is stacking shops that are close to each other so that you don't have to spend too much money on gas. An example would be a simple bank teller shop, where you get paid $10 for each branch of a bank, make a withdrawal or deposit, and rate the teller's customer service.
If it's a popular bank, you can schedule up to ten shops at ten different branches along a route. That's $100 you pocketed for your trouble. Just make sure you take good notes and don't mix up your data from branch to branch.
You can fix a full schedule for yourself each day by accepting shops with different companies. Most mystery shoppers sign with at least fifteen different companies.
You'll need to give yourself time to do the shop (most companies want you to spend at least thirty minutes in a place). You also need time to jot down quick notes in the car from the shop, or fill out printed forms to input on your computer later. If you wait too long to write down details about the shop, you'll forget. Just make sure you park your car far enough away from the place you conducted the shop. Otherwise, you're busted.
If you shoot for making $100 a day, that's $500 a week and $2,000 a month, not counting expenses like gas money, Internet costs, and, of course, paying your taxes. (Even spies are taxed on their income.) You'll have to work about ten hours for that—including driving time and the time it will take you to write and enter the reports.
Clearing $14,000 a year at mystery shopping is considered a haul. Nonetheless, that's well below the poverty line, by the way, and not quite the get-rich-quick career you might have been thinking of (source).