I think you're being reasonable.
Perhaps he could tone it down to a generic headphones of that? like having your son build up to that cost maybe? Or give he could give him money to put towards the headphones... like saving up for them. and if he spends it, then he wouldn't get the headphones that cost that much... or maybe have only $40 or $50 in something to him, and then have the rest held back, but having it to use on them, but not letting him know that. therefore he would have to save for them if he really wants them, and as through a month or so, giving him a little of the money each chore or something?
maybe coming to a middle ground with your ex? like say he doesn't get the $170 expensive headphones, but he doesn't go all the way down to $50 either? maybe (i'm bad with math ), but maybe .... whatever's halfway between 50 and 170 (cant figure it out in my head lol.) ?
And whenever he gives something like this, it would be something kids will usually expect even more the next year (Because it is as much wanted as the headphones are this year, but it will be something more advanced from what it is this year, making it more expensive). Also if he has a sense of getting some money to help him buy them, and has to work or be good, or something on a reward system to get enough money to buy the headphones, not only would he be able to buy them, but he would have more appreciation for them when he does get them, and he will likely try his best not to let them break or anything, and will use them much more than if he just got them. ?