Cost of a child

I've been trying to work out how anyone can seriously believe that it costs $410 a week to keep a 14 year old boy with limited extra curricular interests and no ongoing medical treatment yet that's the figure my ex and her partner are using and the one that CSA apparently use.

Apart from the obvious of food clothing etc which I'll address in a moment there two figures mentioned by my ex and her husband to justify this.
An extra $35 a week in rent after moving to a larger home
Adding our son to their health cover, those with a few minutes on their hands may care to hit iSelect and get a couple of quotes, one for couples health insurance and one for family cover. For those who could not be bothered all the policies I saw listed were exactly the same price for both.

My estimates on weekly costs (not quite what I did as I kept a phone plan for him, tried to make pocket money earned, grew some food, he avoided haircuts and I was fortunate that a friend with an older boy gave a lot of good quality casual clothes). The figures are approximate, no doubt they can be run up but savings can also be found with some care.

$60 food
$5 clothes
$5 grooming
$15 pocket money
$10 extra power & cleaning products
$3 X-Box online
$15 school costs (depends on how uniforms & books are treated)
$10 extra fuel for run around (rides a bike to school). Not much extra curricular
$15 out of pocket medical
$15 holidays
$5 gifts
$5 wear and tear (things that wear out and may need replacing)

Eg around $163 a week. My gut feel had been around $150 but pocket money was not often earned.

Sometimes there may be extras but they are not guaranteed and sometimes there will be savings.
In my view the optional extras should be subject to negotiation, not have the money just taken and used by the other parent without review.

I'm also struggling with the type of ethics that allow for someone who didn't contribute to any of the basic costs while my son was living with me to use CSA in such a brutal manner. The hardship this is causing me has been casually dismissed with a call that we all should get on with it by my ex's partner, meanwhile I'm searching around trying to work out what costs I can cut and still keep some kind of life. I need to get over that and accept that I'd badly misjudged my ex's partner.

I'll probably need to get rid of my aquarium, it uses power. I won't be able to keep using a great personal trainer I've been working with for a while. Discretionary spending on hobbies is almost gone. I'm pondering if it's worth the entry fee and fuel to do another triathlon in April whereas before this started those were not a stretch, now it's two or three weeks of discretionary spending. I'm doing a more difficult drive to get home from work as the Clem 7 is more than I can afford. Tried the train again and the few dollars a week that might save don't come close to the lack of comfort and extra time taken but that may change when I don't have free parking.

I'm trying to work my way to a focus on more positive things, accepting that this is really being done and nothing much I can do can change it. My PT has suggested that I do the Circle of Influence exercise which makes sense, there are some things I can control and I need to work with those.

The CSA system badly needs some kind of independent fairness review process, what they do now is a massive abuse.