That wont buy you a Rolls in cash, but it will keep you comfortably in a revolving parade of high end Mercedes, BMWs and Porsches.
Especially if you're single.
You have a point, but it's worth considering that as far as vampires go she's really young, and that the culture isn't necessarily the same in terms of her options and access to personal resources.
The freedom and lifestyles of Thomas and his siblings aren't necessarily representative of the average white court vamp, since they have relational politics on their side to at least some extent.
It's possible that she's high enough on the totem pole that she isn't easily capable of screwing off like the ones that live in the undercity are apparently allowed to (in contrast with Thomas' crazy sister), but low enough she's still basically a goon that needs more social leverage to get personal luxury resources instead of needing to justify how she spends house funds.
Alternatively, if she is actually the white court version of a baronet or similar, she could be trawling for cheap but fancy cars to class herself up on a budget by accessorizing her goons.
There's a lot of speculation in this line of reasoning, so I could be off in a number of ways. However, I think it's overall more likely that this is something simple and direct rather than being some sort of plot.
To be fair, that is still mostly the case - with eternal suffering in place, growing old and 'dying' is an annoying inconvenience, not a cessitation from existence. Imaging residents of this place learning that the elderly outside dont get to be young again once that irksome period of old age passes once again. They just stop existing forever. Horrific really.
True, but the contrast is higher if they don't age or if age doesn't actually cause decline (As in, they get marks of aging, but it largely comes down to cosmetics and an indication that you've got a lot of experience).
Going through a lifecycle and reincarnating in addition to the eventuality of knowing someone who chooses true death as the centuries go onward gives them a lot of context to work with. It makes the problem something close to the scope of their experience. A condition they've felt, but hasn't stuck yet.
I think the interplay of coming to terms with permanent and inevitable diminishment would be more striking and fascinating to see.
Especially since on an emotional and cultural level I think such an environment would blur the lines between demon and mortal, which would suddenly be in some way felt again by their introduction to the mortal world.
Which adds flavor going both ways; imagine a wizard interacting with a guy who's married to a cybernetic spider demon. Who not only sees them as a normal person, but until recently didn't really think of them as intrinsically different in the way Harry thinks of say Fetches.
There's a lot of space to play with demons and humans that functionally see themselves as different ethnicities within the same fundamental type of being.
Reincarnation has some of that, but the more you play with the effects of age and death the more those lines can be blurred.
There are stories out there that deal with this sort of theme, but the idea of a society that formed without the foundational problem of mortality discovering that it was waiting for them all along is really interesting to me.
It has a lot of subtle differences from a stories about innately immortal groups, groups that used to be immortal but lost it, and societies that mix immortals with mortals, that give it a lot of potential. In a way it even draws pieces from each of them into a novel configuration.
Edit: autocorrect issue