We use a company called Shop Angels for our mystery shopping checks.
As a company we pay them between $45-60/visit, but i have no idea how much the actual shoppers get paid. The shoppers they send into our stores have to be in there for a minimum of 15 minutes and have a 40 question questionaire that they fill out. They are also asked for some personal comments on their experience. Questions range from clean windows? through to whether they felt comfortable in the shop and what items they were encouraged to buy.
In return we receive a 3-4 page report on each visit.
I personally signed up to an agency years ago, but never got sent any assignments
. From memory they paid between $10-20/visit.