People want hundreds, even thousands of articles written for them for Adsense websites, but they don't want to pay an arm and leg for them and they want them to be quality.
I know, I know, tough crowd, but the money is there if you are willing to put in the hard work.
Personally I would charge around $4.50 per article with a minimum of 10 articles required for a purchase.
The articles would usually be around 350 to 500 words in length each and would be based around the keywords they supply you.
You could easily pull in $500 to $700 a day working 14 hours a day with no outlay on your end apart from domain name registration and hosting.
Marketing would be free, just let people know in marketing forums you have this service and people will swarm all over you.
Make sure not to promise too much or get greedy and take on too many clients!