What is the term used to describe a victory achieved only after great cost or loss?

Dear mcneill,

The best term to describe a such victory would be "Pyrrhic victory".

This kind of victory is named for ancient Greek king Pyrrhus of
Epirus:
"Pyrrhus inherited the throne of Epirus in Northern Greece around 306
B.C.E., and as a young man proved himself on the battlefield again and
again. Pyrrhus apparently had great strategic skills, but he also had
the reputation of not knowing when to stop. In 281 he went to Italy
and defeated the Romans at Heraclea and Asculum, but suffered bitterly
heavy losses. The devastation led to his famous statement, "One more
such victory and I am lost" -- hence the term "Pyrrhic victory" for
any victory so costly as to be ruinous."

Source:

Who2: Pyrrhus - Warrior King
http://www.who2.com/pyrrhus.html

Hope this is what you were looking for!
Best regards,
Scriptor