posted by in Greece forum
http://en.greekcourses.uoa.gr/ these courses are from the University of Athens. I think the courses+the exam are about 200 euros. I took only the exam so I can't personally testify as to how good the courses are, but a friend of mine spent a year there and she was very happy with the results.
I have also heard of a school at the center of Athens where courses cost about 30-50 euros, but I don't have much info on it...
Many private language schools offer Greek lessons for foreigners, but they are more expensive than the two I told you about and I believe the results are the same.
If you have trouble researching yourself, I can help you with finding information that is only available in Greek :)
I believe on the bigger islands like Crete or Cyprus it will be easier to find a Greek tutor.