First you'll find on Upwork or Textbroker no problem. Latter will be a long search for a great writer who creates exactly the work you want.
Writing is like most other unregulated professional services in that costs vary wildly, as does quality, and the two are not necessarily correlated.
Get samples, ask friends for recommendations, find blog posts you like and e-mail the authors, give small easily completed sample tasks that grow into big assignments and build a solid relationship with one vendor over time.
What you heard about those sites is incorrect-- or, more precisely, incomplete. It's like my saying "A lot of the people on GitHub couldn't build a program to display 'Hello World!'"
Many of the writers on Upwork who bid on jobs are worthless-- people who have trouble writing a coherent sentence. Others are fronts for offshore content mills.
You will also find (1) copywriters who got thrown out of work when their ad/PR agency folded, (2) reporters who have been laid off as their papers shrunk, (3) staff writers for magazines who quit to have kids (or, if they're men) raise a child, (4) retirees who need cash, (5) people with disabilities who need to work from home or (6) someone working 40 hours, but looking to moonlight to make extra cash.
Unlike (say) database administration, where you can't judge competence from a resume, you can evaluate a writer's ability in 30 seconds-- ask for a writing submission and read what they write.
The $300-500 places price for one job-- that assumes they do it, you go away and they never see you again. Part of the cost is the research time they need to become knowledgeable. For people who can provide a steady income stream, they charge a lot less.
I think it's simpler to find one of the many people who need work-- and would kill for a chance to make $100-$300 a piece-- but it's up to you.