The cost is potential health risks associated with taking the vaccine/booster.
We don't know exactly why myocarditis rates are elevated and some people would rather wait for more information.
Sudden cardiac arrest incidents increased in 2020 (vaccine was not officially realeased until Dec. 2020).[2]
I added the second link to show there may be a rise of cardiovascular disease not due to the vaccine. I'm interested in seeing data for 2021.
[1] https://www.who.int/news/item/26-05-2021-gacvs-myocarditis-r...
[2] https://www.heartrhythmjournal.com/article/S1547-5271(21)007...