Earlier quoted context omitted.
You could get productive in Rust in 3 months max, and then the productivity gains would quickly outweigh the initial investment
Rust code takes longer to write than C code
I don't have a similar evaluation for C Rust, but I suspect the result would be even more skewed in Rust's favor: sure, you might be writing individual lines faster in C, but the lack of iterators, generics, traits and RAII, and the relative difficulty to use dependencies means that you will waste a lot of time reinventing the wheel, often suboptimally.