Tbh, it wouldn't have occurred to me to pin tinsel, etc, onto an existing jumper.
For a start, my DC don't own Christmassy coloured jumpers (they're in to animal print) and for another, all the Christmas jumpers you see are ones with knitted/appliquéd patterns/pictures on. The cheapest ones we were able to find were about £10 each (x2dc).
Our school also asked for £45 for a term of swimming lessons (1 term of lessons each academic year), so £90/yr. School trips, one per 1/2 term (so 6/yr for each child), ranging between £7 and £28 per trip. Even at an average of £10 per trip per child gives £120/yr. PGL in yr6 £295 (this was already the subsidised rate) for 2 nights and 4 to 6 activities.
School non-uniform days £2 each, and there were at least 4 per term (superhero, book characters, Easter, Christmas, jeans for genes, children in need, red nose day, support cancer day, St. George's day, St. Andrew's day, St. David's day, Victorian day, own national outfit day, European day and many more over the course of an academic year) so 12+ per year at a rough total of £24/yr plus an average of £8 per outfit per child (£192/yr) .
Add this to all the plays/parties (usually 3 of each per year), donations for fetes, bake sales, etc and it really adds up ( maybe £100/year including buying cakes, etc at the fetes, etc) At the parties you were expected to provide a plate of party food and drink (in addition to school lunch/dinner) so maybe £2 to £2.50 per plate of food/drink ( gives £12 to £15 per yr for 2 dc) and the plays had an entrance fee ranging between £1-4 including paying for the child in the play (again £3-12 per year per person in family of 6, but tickets limited to 4: gives £12-48 per year).
All of these non-uniform days would have a dress or colour code and it was invariably not a colour we owned ( think bright yellows, reds, greens, blues, etc). So something would have to be bought each time usually at a cost of between £6 to £22. It would never fit for the next year as my DC grow out of things very quickly (very tall and getting more womanly, etc).
Over the course of 6 years for 2 dc this adds up to £4000+ and doesn't even include things like spending money for trips, school uniform, shoes, plimsolls, PE kit, special school bag, school dinners or packed lunch, etc. This was 4 years ago now too! Mine are now in secondary school.
I feel your pain OP.
Secondary school isn't as bad in that way, but there are a lot of other hidden costs which you can't avoid:
Eg, pupils are charged 20p per photocopy for any text needed for their homework or lessons.
They must wear a lanyard with a plastic card holder for their ID card to get around school and locker key attached. In their 6 years there they only get one lanyard/one card holder/ one ID card/one locker key.
The problem is that where it is used constantly to access rooms/open doors it wears away and so the plastic breaks regularly, the ID picture fades and the lanyard clip doesn't last.
If any of these things break/get too faded/worn you have to buy new ones at a cost of: £1 per plastic holder, £5 per key, £10 per ID, £3 per lanyard (has to be school colours). My DC aren't particularly clumsy and are fairly careful, but so far it's cost me around £30 in replacements over 4 years. Food is more expensive, it's around £5/day per child (minimum) for a proper meal, pudding and a drink (ie, not just a slice of pizza or a bacon butty). The cheapest main meal is about £3.50 and that's without a drink or pudding.
There aren't so many trips/year, but they're more expensive and it always says that it's part of their learning for their course, so they have to go otherwise they fall behind. School uniform is very strict and fitted so there no growth room. I'm lucky if their blazers last one academic year as they're very fitted. The entire uniform can only be bought at a particular school uniform shop and it's not cheap. The trousers have to be a certain stye as do the shorts, shoes, socks, etc, and you are sent home for non compliance. Trousers cost £35 each, skirts £28, blazers £50+, blouses are 'cheap' 
at £28 for 2. School jumper is £35, socks are £4 per pair and the rules for their shoes are so narrow that it's hard to comply (impossible when trying to buy cheaper shoes), especially when your dc have big feet. Their latest pairs cost me £70 each and they won't last the year as they're growing so fast.