you don't have to build those caverns as they already exist and we have extracted gas and liquids from them, also the volume should be already known as we know how much we pumped out (unless it's an old site)
Isn't the complexities related to making those caverns air and water tight for long period of time for long term energy storage requirements?
They don't need to be air/water tight for long term, just tight enough with a known leakage factor.
But how cheap is it to determine the leakage factor? I suppose they can just pour in the equivalent water and see if it leaks?
Not much, technologically this is a conventional mine project, only that you mine air volume instead of rock and have to take care of the cavern being reasonably airtight afterwards.
Also you could do it underwater if you live near a coast. Basically an upside down cup with a pipe in it, sunk as deep as possible.
The same company has actually done this[1] with the city of Toronto.
The video is from 2015, and says it was a 2 year pilot project. Do you know if there are there any updates on how the project went? Is it still operational? Did they build any more?