I've been doing display/store audits for a couple years now, but with a major life change in the last 5 months (my dh passed away and I'm not feeling up to finding a full time job at this point), I have opted to try MS. It is true that there are jobs that aren't cost efficient. I do take into consideration the time it takes to study the materials, travel/gas to/from the location, execute the MS and then complete the report. I don't print all the materials, because that will eat into your budget. I do upload the material to my iBooks and save in my DropBox account to reference. I will print the first page of a job to write down initial information and add notes. I always record my MS using Voice Recorder on my phone to refer back to while writing reports.
So far I am averaging about $500/month and anywhere from $15-$30 an hour - that's better than getting a minimum wage job! Yesterday was the first day, I was suppose to make $60 for completing two of the same MS at a bank - a new MS to add to my experience for future MSing. Unfortunately, at the first bank the person wasn't there (a wasted 30 minutes) and the second one I had to get an appoint for an hour and half later (the company paid me an incentive for the first bank for my initial visit and also to return to the second bank). After gas, travel time, meeting with banker and completing the report, with a $44 pay for the one shop, I calculated it paid me $10 an hour including the travel and gas. Win some - lose some.
Overall, I enjoy the varying challenges, learning new things about each shop, interacting with people I don't have to deal with on a daily basis, the flexibility in my time, being able to be home for kids when they are here and need me for rides etc., and the ability to be my own boss.
I have also set short and long term goals for myself (in writing). I hope to be able to make $2000/month by the fall. This will help cover all my budget needs and help me and the kids to stay in our current home.
Finally, as someone else said, I am paying estimated taxes quarterly this tax year to cover my MS income goals and not to be penalized at a higher tax rate come next tax season.
Thanks to the MS Forum for your excellent comments, suggestions and leads, as I branch out in my Mystery Shopping experience!