As with most things mystery shopping, YMMV, pun intended.
Gas station shops are my bread-and-butter these days, and I do remote loops for all but one of the major brands. I would not do a route without them, so I don't fret too much about gas expenses. My goal is always to come home with more than I started with, but that's not always the case.
I also look at things from a cash flow standpoint, so I'm all about gross pay when taking assignments. With the MSC paying weekly (or better during the holidays, yay), I can keep my thing rolling.
Analyzing my latest activity:
I just completed a couple of loops over the last few days. Mostly Yellow Crustaceans, with a Red/White/Blue Conditional Reveal and Red/White/Silver/Sometimes Orange&Blue, and a couple of meals (Dine-in Chinese fast food and Drive-In Stall burgers). All were heavily bonused.
Total time including driving/working was 26 hours, and I had a hotel night for $51. I'm not going to factor in the beers and snack I had after working, because I'd be doing those regardless.
Net after deducting the hotel comes to $785, or about $30 an hour. I did have to buy some gas to get home (this was really remote, into BFE Wyoming), but I'm still close to $29/hour, and I haven't even figured out my mileage yet.
Well worth it for me, plus I got on the rewards program for Old Chicago, so a free appetizer for me on my next loop through there.
Have synthesizers, will travel...