Yes, those are 2011 numbers, but far lower salaries suffice to carry the argument. (If Hermione lives for another 170 years, she has to pay Harry just $20K/year to repay him.) Also, this plan does have her working in finance in 2011 (when she's ~22, she can't start that much earlier.)

It does set a bad precedent, but I wasn't talking about that - and this exact situation (completely legal, Harry doesn't want to destroy Lucius) is unlikely to ever come up again. (Future challenges of this kind could plausibly be met by having a Dementor eat the kidnapper, optionally after paying the ransom.)