I mean, that's already what they do with sex workers
Between hospitals blood is bought and sold at the price you mention.
Radiolab had an episode about this some time ago.
I feel like the market can find a solution for this that's unworkable for blood banks because of regulations, hospital requirements, scale, etc.
i.e., force applicants to prescreen their blood and permaban them for lying or if their blood fails drug/illness/etc. checks.
And if we are really thinking about catering to the uber rich just raise both prices by an order of magnitude and you'll have plenty of donors and probably a ton of buyers.
But for some years, a lot of prostitutes from Eastern Europe got brought by the mafia and drove the prices way down for everyone. As a result, the market is now filled with exploited girls, working conditions are awful(ler ?), and the libertarian El Dorado of 'young women voluntarily selling a service' is dead, replaced by a dystopia of what has basically become selling drugged meat until the meat is dead or not fresh enough to be sold. The meat comparison is unfortunately not that much of an hyperbole, just go for a jogging in Bois de Boulogne and you'll see true human misery.
I'm afraid the same kind of scenario is not that much of a stretch for blood. It could even be worse, as a prostitute have to at least be presentable so bad treatment has to be a bit restrained.
I think it might make more sense for super rich people to pay people directly to give them their blood, which is morally questionable but maybe not particularly worse than, say, paying for a surrogate mother? It sounds a lot less traumatizing at least, both physically and psychologically. After all many people do donate their blood for free several times a year.
I'm pretty sure you could convince many young people to give up some of their blood for a pretty cheap price actually. No need to resort to kidnapping and slave farms.
Many young people already do give up their blood for a pretty cheap price, so I don't think there's any reason for this to become a black market thing.