I realize this thread is old, but in case anyone has landed here asking the same question. A great many services have sprung up recently that offer very cheap "scientific English editing" services, but you will find that the quality of editing provided by these companies is poor. A qualified scientific English editor should have a Ph.
and academic publications, and ideally should be a native English speaker. This means that most qualified editors are professionals who live in the U.
, Canada, or Australia-- all countries with a high cost of living. What does this mean for you, the customer? Essentially, if you are paying less than 5 US cents/word for an individual editor, or less than 10 US cents/word for a company, you are not getting a qualified editor. There are few exceptions to this rule of thumb, I'm sorry to say, and I do understand that this price can be quite expensive if you are from a developing country.
However, you will not be saving money by hiring an unqualified editor-- instead, you will be wasting your money. One of the best ways to determine whether an editor is qualified is to ask for a list of their publications. A qualified editor will have at least five publications in respected academic journals.
Even better is an editor with both academic publications and popular science books or articles. I can recommend a few services that I know provide high-quality scientific editing: JetPub Scientific Communications, Ruff Draft Writing, and BioEdit UK, as well as my own service, Carpe Diem Biomedical Writing and Editing (links below). There are many more out there, though-- look for clear, posted qualifications and a list of publications, and you should be able to find one with minimal difficulty.