he really likes music, so we're thinking of a music teacher and piano, stuff like that, but it seems out of our price range at the moment
YABU. Very.
I'm one of 6 children, and various of us did various activities. As I grew older, I came to realize how much it all cost, and pulled back from an expensive activity because I felt guilty I was costing so much, even though I really wanted to keep going. How much we all "cost" wasn't thrown in our faces, but if you're halfway intelligent & sensitive to the family as a child, then you come to realize.
And one of my siblings seemed to get whatever they asked for.I'm sure I would have too, if I had asked, but I felt guilty to.
It may be that your other DC aren't interested in external activities, or expensive ones at least! But it could be that they'll feel they can't ask, because they've picked up the limit to family finances.
It's a real mistake to think the way you do in your OP, IMHO.