She has had a vast river of gold pass through her fingers and not had it stick in a way that'd you'd be able to clearly say that yes, she has it. It's been spent on books that are now in a library she controls but does not own and research materials and infrastructure with at minimum dual-use.
Her idea of going on vacation is to leave for a few months to murder dark elves and even her very nice home doubles as part of a superweapon. It's complicated, in the best way.
This.
Mathilde has repeatedly has opportunities to acquire great wealth and freely passed it up, usually without hesitation. She consistently sees wealth as a lesser temptation than things like knowledge, research opportunities, or accomplishing a good deed.
Even in a case where she could have kept a good deal of money she had already considered freely given (the jewels from Borek before he disappeared), she decided to give that wealth to Gotrek's widow rather than take the alternative options of compensation even when no one else would know or care. That's the Vow of Poverty mentality--that the appeal of money is greatly lessened beyond practical uses and good causes.
Remember, Mathilde discovered a huge pile of treasure in Karag Nar when she infiltrated it and could have taken anything from it and no one would ever have known, but she never even
considered it. She laid down on it for a bit for fun, but the appeal of acquiring any of it was absent--instead, the appeal was on accomplishing the mission and doing the right thing.
It's not that she's poor--as we've seen, the Vow of Poverty doesn't actually mean living in poverty. It's that she isn't tempted by money beyond a "is there a practical use I can immediately see for this", and if the answer is "no" then it becomes a
burden in the "I guess I'll have to find some way to deal with this, ugh" sense.
It's telling that the transcendent boon she got from Belegar was spent on building a public library for everyone that would survive long past her own lifespan--not named after her, either. It's not a matter of legacy, it's about seeing a tragic problem in the world and wanting to do what she can to fix it.
Heck, even the latest gambit to buy up a bunch of ithilmar and sell it to Laurelorn was used to secure a book deal with the Library of Mournings rather than just straight wealth.
The assassination of the late Tsar? Asked for a project that would benefit both Kislev and the Empire, not herself.
The temptation of money is not absent, but it is remarkably limited and vastly overshadowed by the temptation of knowledge, good deeds, or magical opportunities (research or material). Which is exactly the kind of mentality that the Vow of Poverty wants to cultivate.
