One, the writer will already know the topic and the research is minimal because he/she would already know the topic and only has to research 2 or 3 papers and this can be done in a matter of an half hour (all from available online resources). They can then quickly skim through it for analysis in the article they will write. They don't have to do everything in one sitting; they can do half an hour here, half an hour there and then write it up over the next day or two. Assuming they are reasonably fast typists, the entire write up shouldn't take that long. This of course assumes they are quite well-trained in medical writing.
I think $400 to $600 for around 10 hours of work each month is good pay, especially for a student, some of whom would be toiling away at a bar or a store for much less with longer hours anyway.