For my son who is rather anxious, this question was of upmost importance around age 10 or so when we were going through some rough times. Essentially, what he was asking us is "are we going to financially be ok? Will I have to move? Will I have enough food to eat? Will I have to leave my school and friends?" Saying things like "we can't afford it" would have turned his anxiety up to 110% - I would really caution against saying that to an anxious kid - that can lead them to believe their life is less secure than it is.
The answer I gave is: Compared to the majority of the people you know, and the majority of the people in the world, we are rich. Compared to super wealthy people that are often showed in movies and on TV - we are poor. We have plenty of money and will be ok, we just choose to wear used clothes and drive old cars (insert other mustascian activity) so we can go on vacations, go out to eat, (insert item of luxury they'd understand).
I also completely agree that comparison is key. Among our social group, most families have similar levels of wealth. But I find going to public school in a diverse school district has been key to his understanding of how others live. He goes to a school with kids who just came to America from a refugee camp, kids on free/reduced lunch, kids who have never lived in a house, or gone on a vacation, or had parents that were married, etc. So while he does know the kids who have all gadgets he doesn't, he also knows some who really struggle and that he is pretty lucky. To me this is one of the biggest benefits of not living in a homogeneous environment.