Hello!
Instead of using prettyNum() to format it, I suggest dollar() from the { scales } package.
> library(scales)
> x <- 2334.12
> x
[1] 2334.12
> scales::dollar(x)
[1] "$2,334.12"
One thing to note, however, is that the object returned is a character rather than a number.
> foo <- scales::dollar(x)
> str(foo)
chr "$2,334.12"