I've never hired anyone from such freelancer sites for a few reasons. First off, freelancers who offer their services in forums like the Warrior Forum understand Internet Marketing and are more likely to know what you're looking for and be able to deliver. Secondly, while there are people who are willing to work for a lot less, I wouldn't trust a Chinese person to write my salesletter (not a comment on Chinese people, I am one :P) when I could hire a native speaker.
There are often WSOs on the Warrior Forum where writers offer to write salesletters for a few hundred dollars. Usually, without such discounts and special offers, you should expect to pay in the low thousands $1K-2K. Of course, there are always Warriors looking for writing assignments in the Warriors for Hire section that charge reasonable rates and usually do very good work. For expert copywriters such as Vin Montello or even Frank Kern, you're going to be paying much more than that.