I think it would depend on how close we were. At the moment I'd only be about to spare about $50.
But if I was extremely well-off and I was very close to my niece or nephew, I might be very generous and put money towards a car or shout them an overseas airfare.
I don't have nieces or nephews yet as all my siblings are still fairly young. Some aunts or uncles are one of the most significant adults in the child's life and others are people who live interstate or overseas who you might see for Xmas every three or four years, so it's hard to give a definitive answer.