My ex has managed to fix his finances in such a way that the CMS made a £0 assessment for our 2 children. Just out of interest I looked how much it would cost me to kit both dc (both at secondary, but not the same one) out with everything the schools say they need.
(Both calculations were based on buying everything from the uniform shop in the smallest available size and enough for a week. So 5x tops. 2 x jumper. 2x trousers. 1 blazer. Socks. 1 full set of pe kit. Shoes and trainers were not included. Plus any extras school say they need, eg lab coat, logo bag)
DS1. £176. He would also need shoes and football boots on top of this. Possibly trainers, I never fully understood the pe footwear thing at his school. ( he changed schools just before lockdown)
DS2. £278. Then shoes, trainers and football boots. Compulsory 'locker and school bag package'.
Then stationery, shin pads and mouth guards if needed. Luckily I have realised that although DS2s school list a specific pair of trousers, in a specific shade of grey, they don't actually enforce it and m&s trousers seem to be fine. White shirts are the same from anywhere so can make a small saving there. The 'compulsory' lab coats are never worn, but most parents buy them because we are told we have to.
Of course both my dc are older and need bigger, more expensive sizes. And even 'cheap shoes are upwards of £30 when they have massive feet.
I asked their Father if he could make a contribution. His reply was "no I have 5 other children to look after". 2 of them aren't his and have left school, one is actually over 18. 2 of the younger 3 are too young for school so they don't have mountains of uniform to buy. But the CMS, and the law say he doesn't have to pay anything so he won't. 
TLDR: uniform is expensive. NRPs are not expected to pay towards it. It sucks.