Me who gets to go to rock/metal shows of all sizes from anywhere to 20-70 dollars a ticket with the average being about 40, but yeah ticketmaster/livenation fees are the absolute worst and make every concert in existence alot more than it should be. But atleast they're being sued.
I don't think I've spent as much on any artist for simply tickets beyond Metallica, but maybe thats just the genre thing.
I'd say go just for the experience and that way your brother isn't alone, as you said the quality time is worth it even if the financial side wasn't worth it.
Concerts are insanely overpriced these days (as far as I know).
I've never been to a concert, but if my little brother went to a concert I'd go with him to watch him (you're being a good sibling).
As for 17 year olds and Billie Eilish, I'm not sure if it's a regional thing or something else because I'm 17 and her name never appears ever in music conversations.
Have fun~!
Discord