There's an interesting blog that quotes salary figures. The blog is called "MD Salaries" and there is an entry about "medical writer salaries."
The difficult thing about quoting salary figures is that the range can be extremely broad depending on your experience, education, background, skills, etc. It also really depends on the industry. The phrase "medical writing" now covers such a diverse field, ranging from creative writing, research protocols and safety reports, news articles and reporting, CME/CE, marketing and promotional documents, and so much more.
I don't think I've ever found an industry that has a broader salary range that the non-clinical healthcare industry for clinicians.