Цену вещи узнаешь, когда потеряешь

See Что имеем, не храним, потерявши, плачем (Ч)

Cf: The cow knows not the value of her tail till she has lost it (Am.). The cow knows not what her tail is worth till she has lost it (Br.). One's blessings are not known until lost (Am.). We know the worth of a thing when we have lost it (Br.). We never know the value (the worth) of water till the well is dry (Br.). We only know the worth of water when the well is dry (Am.). You never miss the water till the well runs dry (Am., Br.)