Both my DDs board - they are 10 and 13. The school isn't close (we are in Surrey and they are in Worcestershire!) but since the eldest started we have had 3 moves, including Germany, so the location was not the most important thing for us.
There are 2 levels of CEA - junior and senior. Senior is currently £5111, as nell12 said. Junior is £3962. They increase every summer but not usually as much as fees increase! Generally speaking, Junior is for prep school and Senior is for senior school, with the switch happening at 11 or 13, depending on the schools.
Do ask for forces discounts - and if you are still struggling to afford it, ask for more. They can only say no and won't think any the worse of you for asking. We currently top up the allowance by £1000 per child per term, although when the eldest moves to senior school in September we will be paying more for her. That is with 20 - 25% discounts!
I think making the decision to send children to boarding school is incredibly difficult and I can promise you will shed more tears than they will! Having said that, our girls are getting a fantastic start to life - one we could never afford without the allowance. They are also becoming truly wonderful, independent people. Sorry, came over all proud mum for a moment!!
How old are your children?