worth one's weight in gold

Because gold is expensive even in small amounts.

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worth one's weight in gold

1. (idiomatic) Very valuable.

   I find my dog is worth her weight in gold on those long snowy weekends when I can't get out.

   • 1917 February 12, “If You Knew What Foods?” (advertisement), in The Independent, volume 59, number 3558, New York: Independent Corporation, page 280:

     This book is worth its weight in gold to the housewife who wishes to feed her family for health—and economically.

   • 2011, "Summer (Where Do We Begin?)" (song), Phineas and Ferb the Movie: Across the 2nd Dimension

     Summer, every single moment is worth its weight in gold

     Summer, it's like the world's best story and it's waiting to be told

• worth one's weight in salt