All I could think of here was "it depends if they time their growth spurts to seasonal changes or not!"
Seriously a couple years back, my son outgrew ALL his sweatshirts between fall and Christmas. I had to get new ones in the middle of winter or send him out with his wrists bare (and it's cold here so that wasn't an option!). He is content with whatever (no brand names needed etc) but finding things that fit is tricky as he's slim. Still, we find things on sale & do well that way, and we happily take shared things from others.
There are so many variables, I can't imagine coming up with an 'average cost'. One idea might be to check out the child support tables: the ones for Canada were 'invented' by looking at many families & seeing approximately how much of their income they spent on their kids. So, you look up your income and then it tells you how much per month you would pay for support, which is supposed to be about what you would be paying for 'kid only things' were that kid in your household. There are separate tables for each province, as the cost of living varies, but in Alberta, for instance:
On an annual income of $30,000 (which would be a well paid administrative assistant, or entry level junior technical person for instance), each month you'd pay $245 for one child, $437 for 2, $592 for 3 and $720 for 4 kids. On an annual income of $60,000 (which would be a manager or senior technical person for example), you'd pay $519 for one, $866 for two, $1144 for three, and $1371 for four.
Remember these are Candian figures - our health care comes out of our taxes, and so our take home pay is usually about 30% less than our gross - but then again, most of us have coverage for things like basic prescriptions (where you copay but most prescriptions aren't prohibitively expensive), and you only pay out of pocket for elective procedures.
It still seems a big high to me, but I haven't ever tried to track it. My kid is just part of the family - I don't itemize the 'cost' of him.
(And he came home from summer camp today and told me how much he MISSED me ... and he's eleven! and a boy! No amount of money can express the value of moments like that

)