First, none of them pay WELL. That being said, some pay better than others. There is, however at least some correlation between shop requirements and outlays and the detail needed in the report to the amount of pay. This is not to say that the $3 jobs are easy, but rather that the $20 and $30 jobs (and more) usually require a lot more of you.
Get your feet wet, learn to make timings and observations routine on jobs where you are out little more than your time if the job gets rejected, and then move on up to other work. While each client and each company is different in their expectations, there is enough similarity that it makes sense to start simple.
There is also the experience factor. With many companies you will not get the opportunity to self assign jobs until you have successfully performed a variety of shops for them. And when you put in a request, if a scheduler has a proven shopper who also requested the job, they are likely to be given the work. With other companies you will not even be aware that they offer better jobs until they deem you an excellent shopper.
Finally, the same clients may not be nationwide. A company who has mostly mediocre shops in most of the country may have a regional client in your back yard that pays quite well. That is the case with a few of my favorite companies. The only way you really will know is to sign up with a number of companies and see what they have available near you. Most companies post jobs towards the end of the month for the following month, so get some shops under your belt to see if this works at all for you, then sign up with more companies and watch the job boards.