DS paying for 1/3 of school trip

[MemyselfI2]

I posted a previous thread about splitting the cost of extras over and above maintenance payments, background being that my DS has a chance to go on a school trip. In my original I was talking about him going abroad. He has now decided to pick a much cheaper trip because that's what most of his friends are doing and he'll have more opportunities to go abroad in the next two years.

Anyway, I texted my ex telling him the cost of the trip (150) and that a £50 deposit is needed and that I'd send over the info. Didn't outright ask for any money, left it open to see what he came back with.

My DS tells me now that ex has told him we can spilt it three ways and that DS can pay the £50 deposit from his account that ex outs £20 per month pocket money in.

I'm not being unreasonable to say that's a shit thing to do, am I? DS is 11. Ex is about to go on his fifth trip abroad this year and gives me £100 a month for DS (up £35 since April), and pays £25 club fee. He's only gave me money towards one other major trip and then told DS to take spending money out of it!

Would anyone here expect their kid to pay 1/3 of a trip at aged 11?

*monthly foreign jaunt even.

My mum and dad did things like that to me even younger than 11. I don't think it's wrong necessarily. I also never got pocket money so it was out of birthday/Christmas money too. I don't think he's probably doing it to be a shit, but it shows he does actually want to do it if it's his money.
My DP gave his DC £5 each in the arcade and funny how when he's paying for everything on demand there are tantrums about it not being enough etc even when they are told they will have x amount to spend each at the beginning. When he hands over cash, they want to save it!

When my dd came home last year with a letter about a school holiday, which cost £260, I explained to her that it was a lot of money to find and that her brothers wouldn't get that so if she wanted to go she would have to save half of the full amount. We worked out that she would have to 'earn' £1 per day to achieve it and agreed that she could do this by being helpful and responsible so she did things like tidy her room, doing homework/reading without being asked, helping/being nice to/not killing her brothers/washing up. We gave her a box and at the end of each day gave her a pound if she had done a couple of things each day. There were days when she did nothing or was horrible to her brothers and she got nothing. The next day she would make an extra big effort to do good and helpful things.

I don't think there is anything wrong for a child to have some responsibilty in saving the money for trips like this.
I don't think its right, however, to tell a child to take the money out of their savings.