Black Panther takes place about 13 months after Age Of Ultron. So in a little over a year, did Klaue manage to go through several billion dollars? What in the s**t could he have possibly done to waste that money so fast? That's not merely "buy whatever you want" money, that's "buy and snort all the cocaine sold in Africa" money. No, literally.
You cannot physically withdraw and set fire to cash fast enough to burn through that much money in 13 months. This is waste on the level of several corrupt dictatorships, not an individual who is hiding out not only from the Avengers, but also the entirety of Wakanda. And he has nothing to show for it! Hell, he even says that the sonic cannon / prosthetic arm he acquired is stolen, so the sum total of things he bought with his money seems to be the sweet vest he's wearing. You'd think a rich supervillain would at least have a better getaway plane. Put your face or something on it, man, damn.
Maybe Klaue still has the billions of dollars, and is just robbing museums with a psychotic ex-soldier for fun -- which, to be fair, is far from the worst thing a sociopathic billionaire could do to entertain themselves. But if that's the case, every inch of his body should have been covered with diamonds. The likelier explanation is that Marvel assumed, not unreasonably, that everyone had forgotten Age Of Ultron.