Liquid nitrogen
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77K. Low enough for "high temperature superconductors" such as yttrium barium copper oxide, but for "low temperature superconductors", you need to go much lower, typically with liquid helium, which is likely much more expensive. TODO by how much?
Where Do You Get Liquid Nitrogen? by The King of Random (2016)
Source. He just goes to a medical gases shop in a local industrial estate and buys 20L for 95 dollars and brings it back on his own Dewar marked 35LD.Making Liquid Nitrogen From Scratch! by Veritasium (2019)
Source. "From scratch" is perhaps a bit clickbaity, but I'll take it.