In my experience, the cost of static typing feels roughly constant per line of code, while the benefits of static typing feel roughly O(N log N) in lines of code or O(N) in number of cross-type interactions. These are just wild guesses based on gut feelings, but they feel about right. The constant factors are affected by individual coder preference, experience, and ability, but also specifics of thy type systems involved and the strength of type inference in the tools being used.
In any case, I think often times dynamic typing proponents and static typing proponents have vastly different notions of what a large code base is, or at least the size code bases they typically use.
One problem is that many code bases start out where the advantages of static typing aren't readily apparent, but re-factoring into a statically typed language is often not realistic, even/especially when a project starts groaning under its own weight.
I'd love to see more mainstream use of gradual typing/optional typing/hybrid typing languages, especially something like a statically typed compiled subset of a dynamically typed interpreted language, where people could re-factor portions of their quick-and-dirty prototypes into nice statically typed libraries.
