Read the 'Sticky' threads at the top of this forum section. The 'Nuts and Bolts' should give you a fair idea of how it works. There are no reputable companies that you pay a fee to shop except if you want "Premier Shopper" status or some such nonsense with Satisfaction Service and I think there may be one or two others. The outfits that want to charge you DO NOT DO JOBS, they only refer you to companies where you can sign up for free. There is a list on this forum of companies where you can sign up for free and oh surprise, the free list here is probably better than the list you buy there.
You are never going to make "Real Money" mystery shopping. After a bunch of years a good month for me is about $1000, about half of which is useful reimbursements. Some folks live in markets with more or less jobs available, more or less shoppers to compete with and are willing to drive further or not as far. These variables alone will start to determine how much you can make.
Further determinations then are how cost effective your vehicle is, how fast your internet is, how easily you can produce acceptable reports, what kind and price of jobs you are comfortable doing and how many companies you build working relationships with. You are your own business and your drive and determination are what will build or kill it.