Douglas Main writes that helium isn’t just used for filling balloons. It’s used in heavy industry and we’re running out:

• Helium-3, a useful isotope of helium cost about $100 a liter for decades.
• After 9/11 the US government used the same isotope to detect bomb making materials, causing demand to spike.
• Prices have increased by a factor for 2.5 each year since 2008, and not only will science take a hit, helium balloons may start to cost up to $100 a pop.
• In 2021 the US will cutoff access to its strategic helium reserves causing prices to further spike.
• On the plus side, helium is fairly abundant on the moon so if the earth ever gets its moon programs together again, we could mine some there.

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Source: Popular Science