The willingness (or reluctance) of an author to pay for a sample edit is a clear indication that they acknowledge that your time is as valuable as the skills they want to employ you for. Many other service industries charge a call out fee to cover the time it takes them to assess the work in question, and a sample edit is no different. Even web developers charge a consultation fee and editorial services are no different. This is why you should not be put off when editors. charge for their samples.