You could put some numbers in a hat, shake it up, and pull one out and be just as close as any of us.. but it really depends on the area, your travel, the fees, and so on.
Set aside at least 15% of all your earnings just in case to take the bite out of taxes. I'm guessing mileage will bring you down on your taxes, but when you're combined with a spouse it could go up drastically.
My last three years were all over the place depending on me.
1) $3,500
2) 34,800
3) $9800
of course, for most of that time, I had a job but I did get laid off the better part of a year last year so I shopped full time. I learned quite a few lessons about balance after separating because I was working too much and being gone from home too often. It wasn't just trust that was an issue -- but 90 hour weeks are not healthy.... We're all good now and I pick up 0-10 shops a week on the way to and from my regular 9-5.
Best thing to do is get into a groove. If you're jobless, at home, and stuck -- make sure you set up spreadsheets to track everything from mileage (also get a mileage book) to expenses and tolls, and all shop receipts, etc. If you were to sign up with 2 companies per day, after a couple or four months, you'd be signed up with pretty much every big name mystery shop company and mystery shop or retail app there is and could see from there what is in your area, how often, what it pays, etc.
Seriously, set aside 15% and don't think of it as cash payments. And if your tax liability gets jerked way down due to mileage, then woohoo

I went negative one year on mileage... actually, 2 years since I didn't include some of the early years working where I was all gungho over $5 fees and $12 in free groceries 10xs a week.
Browse these threads and scoop up all you can. Oh, and be prepared that there can be misinformation and cranky humans behind some of these keyboards.
G'luck!
PS it's more fun with a glass o' wine in your hand.