The billion dollar mistake refers to Hoare's statement, not the fact that it might have been a more or less costly error. And errors in terms of security are not all that matter: a simple crash can cost money too.
https://en.wikipedia.org/wiki/Tony_Hoare#Apologies_and_retra...
In contrast, [14] Null Pointer Dereference only has a score of 9.74.
This is why I disagree that null pointers were the billion dollar mistake - buffer overflows are.
C's Biggest Mistake: https://www.digitalmars.com/articles/b44.html