I'll say this, from my own experience - I did do a few projects through a site that used to be called Get a Freelancer. I did the projects for WAY below market rate, but I was new and wanted to get some experience. I gained one long term client from doing so, but of course he was always my lowest paying client, having started out so low it was really difficult to get his rates up and after a time he could really no longer afford what I was charging. And I could no longer afford to work with him
I don't regret it, not sure if I would do it again - especially now that there are so many more offshore service providers on those sites bidding at extremely low rates.
With that said, I have also assisted clients in managing hires through those sites and in our case the lowest bid was rarely, if ever, the one selected. It was about economy to some degree, but quality was more important.
So I do think there are some clients who are willing to pay more, but they are few and far between.
There are quite a few thread in this section on the various freelance sites, here are the
search results for 'elance' - perhaps you'll find them interesting or useful