Final answer:
Greece received $400 million in military aid from the United States as part of the Truman Doctrine, after Britain ceased its financial support in 1947. This funding was vital in helping Greek monarchists defeat the Communists.
Step-by-step explanation:
The amount of money that Greece received to help the monarchists defeat the Communists was substantial. After World War II, Great Britain had been providing economic and military assistance to the authoritarian Greek government to bolster it against the Greek Communists. However, that assistance ceased in 1947 when Britain could no longer sustain the financial burden. Subsequently, the United States stepped in by enacting the Truman Doctrine, which led to Congress approving $400 million in military aid to Greece and Turkey. This aid was a critical factor in the defeat of the Communist forces in Greece.