You don't say what you earn, but using your analogy, let's say you earn £1.5k a month, but your friend is on the dole....would it be okay, if every time you went out together, she expected you to pay for her, simply because your income is HUUUUUUGGGEEE compared to hers?
Honestly, some people seem to be hard of thinking.
The Ex should be paying half of the nursery bill, plus extra for the other stuff on top. I can only presume that CMS don't look at nursery fees and include them within the calculation, because there would be many mothers who would insist on sending their kid to a nursery that's way more expensive than the norm, and how could you hold a Dad accountable for half of that, if he doesn't get input into which nursery is used? Same for clubs that the Mum could sign the kids up for.
However, Op, remember that the CMS figure is the bare minimum he should pay - could you negotiate with your Ex and say "look, we both decided on this nursery, you need to pay your half, plus extra on top for all the other stuff". Would he be open to that?
Good news is that you will be quids in though, once your child goes to school. So that's at least something. But YANBU, of course!
The fact that Sharon down the road only gets £7 a month from her Ex, has no relevance whatsoever. I can't believe that anyone would think that has anything to do with it! Weird!