I'm guessing that if you live in a area with affordable rent, your options for high-paying shops may probably be lesser than urban areas. I think it would be relatively easy to make $500/mo shopping in Los Angeles, but the expense of living here is much greater.
If you have an option to find additional work in another field, I think that may be a good option if paired with shopping. I find it MUCH easier to take shops that offset my other living expenses (dining, groceries, dry-cleaning, pet food, oil changes, etc.) than to directly earn the equivalent of that in shopping fees.
Also, keep in mind that if you are working FT as a shopper, it may drive your living expenses up. Many shops require a car and lots of driving! Add to that the internet, printing, scanning, faxing, photo and computer requirements and it gets expensive.