度胸 is the general word for courage or nerve. 度胸千両 is an emphatic, idiomatic compound that intensifies the meaning by comparing courage to a large sum of money.
肝っ玉 is a more colloquial term for guts or boldness, often used in phrases like 肝っ玉が据わっている. 度胸千両 is more literary and proverbial.
Etymology
Compound of 度胸 (courage) and 千両 (a thousand ryō, an old currency unit). The phrase figuratively equates boldness to a fortune, suggesting that courage is as valuable as a large sum of money.
