There is no right o wrong answer to your question, and you will probably get a multitude of answers. I will provide you with my answer, knowing full well that it is truly right only for me. My first test is whether after subtracting the IRS mileage rate, will I still be making any money. Even that is not an easy test unless I am doing just that one shop, then the round-trip mileage is easy. Otherwise, I have to guestimate based upon the likelihood of being able to share mileage among shops on a route. IF I am being reimbursed for something that I truly need, that gets considered, particularly since if I am being reimbursed (not part of a flat fee), each of those dollars is worth more in terms of pre-tax value. IF I am being asked to buy an overpriced item at a convenience store that I would pay less for at a supermarket, I factor that in. I personally value fine dining reimbursements in terms of 1) would I really like to eat there, 2) can I afford the meal if the shop goes sour, 3) is the amount of work required (in terms of time) reasonable on an hourly pay basis compared to the value of the meal?
Back to the IRS mileage rate test. I decide that I am making money after subtracting mileage. Then I consider round-trip drive time based on Google Maps. That is objective, if somewhat optimistic. There is value to time. IF I am driving, I am not doing anything else to earn money. Decide how much money you want to make per hour. Divide by the value remaining after subtracting mileage. Suppose you want to make $10 per hour? Would you? Now estimate (or maybe you know based upon experience) the time required to prepare, perform, and report the shop. Add that time up, and add to the drive time. Divide the value after subtracting mileage by that total time. Is the pay per hour enough?
This sounds cumbersome. Early in my mystery shopping days, I realized that I was making very little money per hour because I had not considered these aspects. With experience, i now quickly approximate. I do look up mileage, because that is objective.
Then there is the issue of how easy and how painful. I like bank shops. Paying me to make a deposit is pleasurable for me. $60 to open another account so that later I can do a teller shop? Sign me up. $20 to ask about a checking account? Sure. But in NJ, gas stations are a pain if you can only be reimbursed for $1 or $2 of gas. Why? Because NJ is full service only. Drive up to a pump, and ask the attendant for $2 of gas, less than a gallon? Painful, maybe even dangerous in the wrong neighborhood. Sure, I can buy $10 of gas and view the $2 as a discount, but then I can only do 2 or 3 shops and my tank is full. $2 of gas plus $1 in the convenience store (which in NJ often means the service station office here there may be nothing to buy) is discounted by me to a value of ZERO dollars. Just painful. In DE or PA, the $2 is worth $2 to me.
$8 reimbursement for a burrito (no reimbursement for a drink)? IF I am a route and need lunch, maybe. I might prefer to pay $2 for two chicken sandwiches and no report than to get a free burrito if it requires 30 minutes to report. IF the time window for the burrito lunch is just an hour, forget it since it really constrains my ability to complete the other shops on the route.
I hope this helps you understand MY thought process.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008