a korean friend says its expensive to study english in america... how much does it cost?

Depends on the country in America

It depends on where in America you want to study. In New York City, which has a few Korean/Asian communities, it's much more expensive than the costs Darth Severus gave. Professional private tutors get about $50 an hour (though a student tutor may be just as good). There are many ESOL (English for speakers of other languages) schools and classes, and their costs vary.

Darth Severus... Isn't that overkill? (I love both Star Wars and HP!)

English? Why not go to UK?

Many Asian Country have good English courses. I met many Korean in Thailand, Malaysia and Singapore sending their children to learn English.

yeah, you'd have to pay at least $10/hour for a private tutor, or even $20 if you want somebody trained as English teacher.

Not sure what prices on group classes are, probably around couple hundred per semester.

You can buy disk to teach you. Why pay for lessons?