Development time against run time against saved time.
I spent nearly two days writing this code. (OK, it could have been quicker, but I'd never done a Perl XS extension before, so there was a little learning curve in there while I worked out how to return a string back to Perl from a DLL written in C.)
It takes 25 seconds to run.
It be run only once, ever.
But it saved huge amounts of time - if the work had been done by hand, it would have been hundreds of man hours, and been very prone to error.