I have tried a few of these services as well as going out and finding my own sources which tends to be very time consuming. I would recommend checking out BlogMutt (www.blogmutt.com/). Their pricing starts at $89/month for 4 articles. They vet all of their writers and have gone with more of a contest approach so each week you have a selection of articles to choose from.
They work with you on keywords and you have the option to request edits or reject posts. Equally important they seem to treat their writers well and pay them fairly which makes for more consistent quality.