The average annual cost of 1 year of mAb treatment is about 100K [1], which jibes with your estimate.
However, that is one year of treatment.
Fully human mAbs have a pretty long serum half-life.
COVID-19 should clear from 1 or 2 administrations, not the 12-24 per year typically seen with other mAbs.
[1]
https://www.ncbi.nlm.nih.gov/pubmed/29461857