Or the T-shirts which seem cheap on the internet until the shipping cost is added. (I once bought one for my son as a birthday present, only to find I had to trek to the local post depot to pay an import tax which was more expensive than the t-shirt itself).
Nor is it the cost of a school trip or the bus home or the train fare to London to see a gig that he really shouldn't be seeing in the first place because it's school the next morning. ('Pleeese, mum. I've done my coursework and it's their last appearance ever.')
It's not even the cost of the way it all adds up when they 'just need a couple of quid' for an evening out and you discover the small change in the box in the hall has somehow disappeared.
No – it's the fact that some sad statistician bothers to sit down and work out how much it costs to run a teenager in the first place. Surely there are some things (if that's the right way to describe a teen) that money simply can't buy?
What possible price could you place on a half-grown kid/adult who makes you roar with laughter (as well as cry). Take the time I got a phone call from a neighbour during my teen's rebellious hair stage.
Did I know, she enquired delicately, that my son had some friends round? At least, she thought that's what they were. All she could see (and hear) through the window was a flock of green mohican crests rising and falling in the moonlight. They were on the trampoline…..
My kids don't just make me laugh and cry. They're good company when they get out of grunt mood. They help keep life in perspective. ('What's the point of making my bed when I'm only going to mess it up again tonight?')
Besides, it's not just kids that are pricey. So are husbands, wives, dogs, houses – in fact the list is endless. But in my view, they're worth every penny. At least, they were until this morning. 'Is that a new pair of trainers?' I demanded as I pushed my youngest out of the front door so I could get to work on time. He nodded. 'I got it off the net with your credit card – you said I could. Remember?'