I totally agree with this (except for the Jehovah Witness part ...

). As I read the OP's vent, the first thing I thought was, "Wow ... way to keep score!" If the 16YO was happy with her birthday party and had a great time, why does mom care? I mean, the OP had it down to how much money everyone spent on everyone and that's the thing that's making her glad she's moving? Interesting perspective -- "I don't like you, but you better send gifts".
I don't know if my parents ever kept score when we were kids -- I had one sibling; my closest aunt & uncle had four kids. Many of our aunts and uncles were unmarried with no kids. Honestly ... with a gun to my head, I can't remember a single gift I ever got from any of them when I was a kid, although I'm sure there were presents. What I DO remember were the wacky holiday parties and the Christmas snowball fights and the entire family gathering around the TV to watch football on Christmas and the "ugliest snowman" contest we had every year.
As for my neices & nephews now? I don't get them gifts based on how much the item costs. I get them things that I think they will like and will speak to their interests. I'm sure I do not evenly distribute the money I spend, but then, the kids know that every single thing I choose for them will be something I really thought about and hunted for and got specifically for them. Neither they, nor their parents, seem to care if little Bobby's present and little Amy's present cost the same.