Personally I don’t think you should be writing 50,000 words for $40, but that’s how I value my time. Some people do offer their services dirt cheap, but people will pay for quality, especially if you have writing samples. That being said, you’re not going to get paid $50/hr and have months worth of work at the start; it takes time to build up a presence on those platforms.
Those are the big 2 sites I know of, but there are many variations of those sites as well. You could also consider doing something other than writing (tutoring, credit card churning, programming, etc.).