OK, here's some data for you -- can't tell you where I ran across it, because I read EVERYTHING, but it was either some sort of restaurant magazine or business magazine: to be competitive, you'd have to charge the customer between $2-4 A DAY PER LOCATION. That's $60-$120 a month, per location. The $2 average is for a straight report: did employee A, B, C, & D each meet corporate standards. The $4 fee would include "full analytics". How many shops a month that fee includes determines whether it's profitable for the MSC -- and the chance that they can find a shopper to perform it at a fee that leaves the MSC a profit.
OK, you got your first client. We won't consider overhead, fixed, or sunk costs for this exercise, for purposes of simplification. He has 3 stores, all within 15 miles of where you live. He agrees to $120 a month, per location, and wants each location shopped every WEEK. He wants a straight report on his desk by 8 AM after the day of the shop, and "analytics" at the end of each month. To keep your costs down, you decide to perform the shops yourself (no law against it; you're in business for yourself).
You just tied yourself down to 12 shops a month, PLUS all the reporting to the client responsibilities, for $360 a month. Or, even worse, you have to find a shopper that will do the shops for say, $8 each, leaving you with $264 profit. Of course, that's only gross profit; if no shopper flakes, you edit all the reports yourself, etc., etc., then it could be your net profit, too.
Feel free to extrapolate.