worth one's weight in gold

Because gold is expensive even in small amounts.

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