Also, think about the "value add" - what can you offer clients above and beyond just writing them an article? Maybe you could provide an end-to-end service where clients give you their niche, you do the research and come back to them with a list of articles you think would be good for their particular needs, and they give you the go-ahead. And you offer to do that for them month after month, so you become like an extended member of the team. Or maybe you offer a package where you write the articles and upload them as a draft post to their site with relevant images so they only have to make quick edits and then publish. The key is adding value. People will pay for the added service if it saves them time and offers convenience.