I'm a public employee in my day job and you can literally google my name and find out how much I make.
That's always fun.
I think it's OK to ask about a salary range for a position, both in mystery shopping and in other types of jobs. It helps gauge whether you are wasting your time applying for it or not. It cuts both ways- Is it too low paying that you wouldn't want to do it, or is it too high paying that you wouldn't be a good candidate?
My day job title contains "analyst," but there are people out there who are "analysts" who really analyze some deep sh|t and make a lot more than I do hahaha, and there are people in my organization with the same title who make a lot less. So when I applied for my job, one of the first things I asked for was a salary range so I knew I was playing on the right ball field.

I think it's OK to ask about a salary range for a position, both in mystery shopping and in other types of jobs. It helps gauge whether you are wasting your time applying for it or not. It cuts both ways- Is it too low paying that you wouldn't want to do it, or is it too high paying that you wouldn't be a good candidate?
My day job title contains "analyst," but there are people out there who are "analysts" who really analyze some deep sh|t and make a lot more than I do hahaha, and there are people in my organization with the same title who make a lot less. So when I applied for my job, one of the first things I asked for was a salary range so I knew I was playing on the right ball field.