You should do some digging yourself and read and watch all the interviews with Dr. Ji. You’ll find the answers to your questions there.
You’ll find the antibody in question can last from 3 weeks in the blood to 2 months up to a year. It all depends on the method they use to engineer the antibody. In this case, the antibody is very potent and provides complete neutralization at a very low dose which keeps the price down and makes it easier to scale. So, it suggests the $100,000 number is drastically off base and ill informed.