Almost all our uniform has to come from a school supplier. I just did a virtual shop as if I was starting from scratch and everything came to about £360. (Included 2 jumpers, 2 pairs of trousers and 4 shirts) but that didn't include shoes, so with school shoes, trainers and football boots that would easily exceed that.
Blazers are around £40, but a nearby (state) schools blazers are over £100 each!! Pe tshirts for my DC are £24, pe socks (which forever get lost!) are £8/pair. My DDs skirts were around £26 each, the other schools skirts are £40 each!
DD had 2 blazers and 4 school skirts that lasted 5 years, and iirc 2 sets of pe kit, some of which I bought second hand. although her shirts were replaced every term at one stage. A pair of shoes would generally last a year, so I don't think I'd have spent anything like that much.
DS has had a growth spurt this year so has necessitated buying new school shoes, trousers, trainers etc (blazer and shirts will be ok until the end of term but hes had for 2 years anyway.
I guess there are all sorts of factors- the cost of the uniform in the first place, how much wear and tear it gets (DS has 2 pe kits because he does lots of lunch time clubs, or after school matches for the school, DD did the PE lessons and nothing else so 1 was sufficient) and how quickly your kids grow. My DC are pretty small so we rarely needed to replace uniform more than yearly or 2 yearly. (Apart from shoes)