Freelancers: Be cautious with freelancers; check their credentials and request samples.
Personally, I would be more weary of company based writing services and their writers. That is because these companies often engage in the practice of bait and switch wherein the quality of the sample they provide does not match the actual output of their services once paid for.
Quality Over Cost: Investing in more expensive, established writers often yields better results.
Again, I have to disagree with this. This is a rule that supposedly works only for the academic writing companies. Established independent writers who have a track record with their clients could offer discounted fees for repeat clients. Prices per page may also be discussed with new clients depending upon the number of pages and difficulty of the work to be provided. Never assume that expensive will remain expensive.