The cost of stopping Hydra was one buck. That's right, just one dollar. In the MCU, Hydra was a Nazi organization that was founded in the 1940s. They were responsible for many atrocities, including the death of Captain America's best friend, Bucky Barnes. In the 2014 film Captain America: The Winter Soldier, Captain America and Black Widow discover that Hydra has infiltrated S.H.I.E.L.D., the organization that Captain America works for. They eventually defeat Hydra, but not before Hydra kills Nick Fury, the director of S.H.I.E.L.D.
In the 2016 film Captain America: Civil War, it is revealed that Tony Stark, a member of the Avengers, is responsible for the death of Bucky Barnes. This leads to a conflict between Captain America and Iron Man, which ultimately results in the Avengers breaking up.
In the 2018 film Avengers: Infinity War, Thanos, a powerful alien, comes to Earth in search of the Infinity Stones. The Avengers try to stop him, but they are defeated. Thanos destroys half of all life in the universe, including Captain America.
In the 2019 film Avengers: Endgame, the Avengers travel back in time to stop Thanos before he can get the Infinity Stones. They are successful, and they defeat Thanos. Captain America is brought back to life, and he and Peggy Carter, his love interest from the 1940s, are able to live happily ever after.
So, the cost of stopping Hydra was one buck. But it was worth it, because it saved the world from Thanos.