Things change. You change the code in response. What broke? Without the tests, you don't know.
"Things change" include "you fixed a bug". Bug fixes can create new bugs (the only study I am familiar with says 20-50% probability). Did your bug fix break anything else? How do you know? With good test coverage, you just run the tests. (Yes, the tests are never complete enough. They can be complete enough that they give fairly high confidence, and they can be complete enough to point out a surprising number of bugs.)
Does that make you pay "at least twice"? No. It makes you pay, yes, but you get a large amount of value back in terms of actually working code.
Things change. You change the code in response. What broke? Without the tests, you don't know.
"Things change" include "you fixed a bug". Bug fixes can create new bugs (the only study I am familiar with says 20-50% probability). Did your bug fix break anything else? How do you know? With good test coverage, you just run the tests. (Yes, the tests are never complete enough. They can be complete enough that they give fairly high confidence, and they can be complete enough to point out a surprising number of bugs.)
Does that make you pay "at least twice"? No. It makes you pay, yes, but you get a large amount of value back in terms of actually working code.