Great article! There are so many differences between the $50 writer and the $250 writer (and, if you target brands rather than bloggers, it can go quite a bit higher).
I also tend to be somewhat of a devil's advocate when it comes to a niche. I've written about everything from cold-water survival gear to how to make your website W3C compliant. If I had to choose, I'd say my niche is business. And that's what I offer my clients that drives my rates past $250: I help them identify a business purpose for each piece of content, and then I write content that achieves that purpose. Once you start asking the right questions, you'll discover that a lot of clients don't know exactly what they want a particular article to accomplish. If you can help them pinpoint that, you're already offering way more than other writers.