Osan charges $300 per written test. In addition to that, they have this weird rule regarding non-Americans taking written tests.
The rule is that Osan cannot administer the test for non-Americans if the number of non-Americans that have taken written tests is greater than the number of Americans that have taken written tests. Therefore there are chances that you may have to wait for awhile before they can give you a slot.
I don't know if Kadena has this kind of weird rule.