It's hard to know for sure but I've heard that some of the paid sites are owned and operated by the same individual and therefore, you might be paying the same person twice!
Personally, I have tried the fee-based ones over the years and have never had any success with them. The jobs are always interesting but no bites. Sometimes all they are doing is fishing ie. seeing what's out there but not really hiring and sometimes in cahoots with the advertising company.
All my jobs were found through other means. However, I do look at some of these job sites and I have access to others. Can be done. It's dog-eat-dog out there and you do what you've got to do!