That varies widely. Some shops only pay you a small amount of money, because you get to keep the product (or eat the meal, or see the movie, etc.). If a shop is high-priority - because it's time-sensitive, or maybe another shopper cancelled at the last minute - you could get a bonus payment. It's unlikely you'll make a living as a mystery shopper, but it could put some spending money in your pockets, give you a free date night, or provide you with some fun merchandise at no cost.