I tried to make a best effort attempt at getting some data on how much co2 was needed - as a baseline just to fill and chill one column. So after a quick gander at the triple point of co2 its obvious that if I wanted to transfer a liquid and not turn to dry ice it must be over 75psi so I did 100-120psi transfer with the tanks upside-down on a scale. Could watch the cold level rise as the transfer took place. Of the little bit of co2 I had to play with, 40lbs wasn’t enough to fill the column jacket but was instead just under half way.
Once in the jacket I let it depressurize so it would get below -50c by converting to dry ice for a general comparison. I suspect most of the loss was from the depressurization. It only stayed at or below -50c for a short period with a longer lasting -35c to -48c ice block.
Price wise, the cheapest co2 I have ever found is at our local grow shop for $0.375 per lb and everything is calculated from this value based on observed consumption rates. No heat load was applied to the columns in any way, this was simply just to get them to -50c or colder to get a general idea of consumption.
It came in right at about $78k per year per column for co2.