Ours are 8.75 and can only be ordered online. I used to buy three from Asda for that amount, and they were really nice too - not horrible.
We fought the school about changing it to this and they won - only as far as the governors though. We didn't take it further.
I've decided that my children will only have two each, as we can't afford more than that, and if they get dirty and have holes in then they will just have to wear them anyway. Luckily my children never seem to notice their clothes being dirty so they won't mind.
The school can enforce it - I did a lot of research about it. Children can be excluded if the refusal to wear correct uniform is persistent.
If they weren't compelled to wear uniform, then mine would not as they rarely get to wear their other clothes, and uniform is expensive, and trousers to fit my son are very hard to find (he's not huge - just an odd shape, so generous ones are massive on him and normal ones always dig in and hurt him) and they wear out easily anyway, I hate polyester.
I've just bought cotton ones and am taking them up by about 4 inches, it's ridiculous. If he could wear normal clothes, sensible ones, like the trousers with elasticated waists he has, it would be fine. Uniform is a crap concept.