I remember having had deals with my parents like: If I pay half, they give me the other half as a birthday present. And my parents could definitly afford it. The neighbour kid, whose father worked for my father after he gave them the place when they sought shelter (Bosnian war refugees), got literally everything he wanted. Our parents explained to us why his parents do that and why they don't. And although we also would have wished for these things we understood.
This is the responsible way to act in my opinion. It shows your kid that they really need to do something if they want something. It teaches them to value the thing that they got and if they spend their pocket money on rubbish it teaches them not to spend money on rubbish. And if it is something that makes sense for them to have (e.g. teaching material, books, a raspberry pi you can still be generous).