Some translators are able to command very high rates but others work for considerably less, perhaps through fear that charging more will make the work dry up. There is a lot of competition in the translation market and like everyone else translation buyers are tightening their belts. Working for agencies does inevitably mean lower rates because of their margin. The best way to achieve higher rates is to work direct, or so they say.
Considering the expertise and experience of translators: detailed knowledge of the source language(s), ability to transfer meaning and write clearly in the target language, detailed knowledge of one or more specialist areas, membership of professional bodies, ability to use translation software etc., good translators should be able to ask for and get paid more.
I don’t need more work, I need better work and better paid work. By better I mean work that fits my specialist interests. If I were paid more I could be more selective about the work I accepted and I could devote more care and attention to it. Result = better translation, happier translator. Win win I think!