Côte-d'Or (a department of Bourgogne-Franche-Comté, France)

plural of bouton-d'or

(literally) gold mine / (figuratively) gold mine, mother lode

(figuratively) golden age, golden years, golden era

golden calf / golden calf (money, wealth)

over the odds, very dearly

plural of poule aux œufs d'or

(informal, figuratively) to be worth one's weight in gold

silence is golden; a closed mouth catches no flies

(figuratively) golden goose (something that generates much profit or gives great advantages)