[Etheryte]

As with anything medical, the price of the item isn't just the item itself, it's purity plus that someone has verified and stamped off on it. I don't know much about argon, but for nitrous oxide, food grade is generally between 99% and 99.9% pure. If you want medical grade, the threshold is 99.99% pure or better depending on what you're doing. Those additional nines are what drive the price up, plus the fact that someone put their skin in the game and signed off on it with their name.

[DannyBee]

So Ultra high purity stamped argon (99.9999) is twice the price of industrial argon used for welding.

Food grade Argon is actually half the price of industrial argon.

See https://www.airgas.com/product/Gases/Argon/p/AR%20FG300 vs https://www.airgas.com/product/Gases/Argon/p/AR%20300 vs https://www.airgas.com/product/Gases/Argon/p/AR%20UHP300

[DannyBee]

In the case of Argon, it's both inert and non-toxic. The main danger is that it's heavier than air (so you can asphyxiate yourself if you try).

As such, "food" grade argon is actually less pure than industrial argon - food grade is 99.996 and industrial grade is 99.998

The practical difference is what contaminants exist - because argon is non-reactive, in food grade argon they are just trying to remove contaminants you don't want in food.

In industrial grade argon, mainly used for welding, almost any kind of impurity can cause issues, whether it would be "safe to eat" or not.