Obviously giving away an organ is far more health-adverse than giving away a small amount of plasma. Unfortunately, lots of people already do donate plasma in exchange for money because they desperately need the money, and that's just for plasma that goes to sick people. In my opinion, if rich people are offering those same plasma donors 10x or more than the amount they'd get from donating to a typical blood bank, then it's at least a big improvement over the current situation. And hopefully they put limits and ID checks in place to ensure people never donate above a certain amount of plasma per day/week/month and risk their health.
Hopefully we'll one day be able to grow or synthesize the youth-preserving compounds in the plasma without requiring the donors.