Everyone's price is different. I don't do this to make money, but I don't sell myself short either. For me, anything I do for less than $15 pay is a waste of my time even if it takes 5 minutes. I won't do phone call shops for $4 or whatever crap pay is being offered. I just did a new restaurant shop and went in the hole for $15, but I think of that as, if I went there on my own, I would have been in the hole $55 and it was an easier report. So the food benefit and the pay was justified for the effort involved. I don't need the money from shopping, but am not going to waste my time with shops that don't fairly compensate me for the time and money. If you have been shopping for awhile, you should easily see fees rise during the month as schedulers need to fill shops. If you have less competition in your area, the fees you get for your local shops will be higher than people in heavily populated areas with a mass of shoppers. The tradeoff, is you have less opportunities to shop than someone in a big city.
I would say, watch the shopboards and get a feel for the high point of the pay for the shops and wait it out until it reaches the point you want.
Because I live in Southern California, I base my shopping on where I will be and what I will be doing and if there is something there than can be done that meets the amount of money that I feel is "reasonable." I won't do shops for ACL or anyone else that thinks they can only feed me and NOT pay me. I used to do some many years ago when I was not in the financial position I am in now. Now, I value my time more.
A buddy of mine shops as well. He thinks that if he works 40 hours per week at a regular job, then your mystery shopping is like overtime. You are working beyond the 40 hours. So the pay needs to meet 1.5 times your hourly rate for it to be worth it to him. I can agree with that to a point, but because my hourly rate is quite high, I would never do any shops if I had to measure it by 1.5 times that.