I think the more interesting cost benefit isn't moving from C++ to rust. Rather it is moving from Java or Python or Ruby or Perl to Rust.

You'd pretty much never make the move from Java -> C++ because of the lack of safety. But you may want to do it because of the other non-safety features.

That is where Rust is interesting and competitive. It's the performance and memory footprint of C++ with the memory safety of Java.