We never know the worth of water till the well is dry.

Related:

• We know the worth of a thing when we have lost it.
  French...
• When the well is dry, we know the worth of oil. --
  Poor Jimmy's...
• Water really has no value...
  until the well runs dry...
• He that does not ask will never get a bargain.
  French...
• Never trouble trouble till trouble troubles you.
  American...
• With enough 'ifs' we could put Paris in a bottle.
  French...
• To compare is not to prove.
  French...
• Fire, water and government know nothing of mercy. -
  Albanian...
• It's all very well in practice, but it will never work in theory.
  French management...