Idioms meaning to do something at great cost

Question

What I'm looking for isn't quite the same as a Pyrrhic victory, as the action isn't necessarily done to obtain victory. Rather, it is any action that will ultimately result in the person taking said action to pay great (preferably terrible) cost either along the way or afterward, whether or not they realize it in advance.

Answer 1

Old timey, but carries the meaning you seek:

Then, as the story progresses, to the supreme sacrificial effort of Jeanie in behalf of her erring sister's life...

Masters of the English Novel

Answer 2

You could try to pay dearly

to suffer a lot as a result of a particular action or event
If you refuse to cooperate with us, you will pay dearly for it.

dear : At a high cost