Yahoo reports that TutorVista offers online, personal, tutoring for pretty much any subject for $99/month, unlimited. The tutoring is provided via Skype, Yahoo Messenger with Voice or Google Talk.
According to Yahoo, the tutors are located in India. However, all tutors are required to have a Masters degree in their subject or have a B.S. in education and another subject. Tutors all are required to speak very good English.
 Today they are having a one day special. Sign up now and get unlimited tutoring for $49/month.
TutorVista is not the only online tutoring service, but I think it has the best price. You can check out these two for comparison.
e-Tutor.com – Online tutoring, learning, and homeschooling resources.