Liquid nitrogen costs something like $0.20 per liter, if you produce it at scale. If you buy if from someone else in small amounts it's naturally more expensive, but probably comparable to the cost of milk.

My question isn't how much it costs to fill the tank in the first place, but rather how much boils off per unit time. A vacuum flask is a great insulator, so it might not be that much. If superconductors are necessary for enough efficiency to make this work, do we lose all our efficiency gains in cooling costs?