Most reputable companies don't charge, you can register with us for nothing, there is an option to do an on-line training module and take a test at the end which does cost £10, but is not compulsory. Look for companies that are members of the Market Research Society (MRS) or the Mystery Shopping Providers Association (MSPA)
The work is very ad hoc, we have over 200,000 people registered on our database!
Some assignments are very boring, others much more enjoyable. For example one of my clients has a chain of 6 famous tearooms in Yorkshire, so every month some poor soul has to go and have a full lunch or tea, and then buy something in the shop .....................