I understand that the boys have greater funds available for scholarships and APs than the girls - making it even more challenging to secure a scholarship/AP at the girls' school.
If parental income is very low (less than £20K) , boys can be offered - if they do very well in the examination - a full fee remission (so 100% of the fees are paid).
APs are available up to a parental income of around £70K. These can be combined with scholarships to reduce the fees further.
The scale is shown via a link from this page: http://www.kes.org.uk/assisted-places.html
If you want to be offered an assisted place (subject to how well the exam goes), there are additional forms to complete on application.
For parents who do not qualify (financially) for an assisted place, the maximum scholarship is 50% of fees.