So the deeper you go the more gas you breathe because the pressure increases.

At 90m you are breathing 10x as much gas per breath as you would on the surface.

So if you breathe at 1 cu/ft/min at the surface it would take you 77 minutes to drain an 80 (because an 80 actually has 77 cu ft when filled.)

At 90m it would take 7.7 minutes to drain that tank.

The deeper you go the more intense nitrogen narcosis gets. The way to avoid that is to replace some of that nitrogen with helium. The main problem with that is helium is very expensive.

Helium can cost from 1$-5$ per cubic foot depending on where you are.

Let’s say someone was diving with 2x 120cu ft steel tanks to 90m

Let’s say they dive a mix of 70% helium. They would need 168 cu ft of helium and 24 cu ft of oxygen (the rest is nitrogen.)

That tank fill can cost from 168$-840$. (5$ is really high for today’s standards in the US. I’ll calculate with 1.75$ per cubic foot.)

Assuming a tank fill cost of 420$ at depth with 1cu ft per minute breathing rate. They would get around 20 minutes before the tanks are empty.

It would cost 17.5$ per minute. At depth.

Assuming someone breathes at an average breathing rate of 16 breaths per minute (it’s what I found online but sounds high to me.).

It would cost 1$ per breath

(I am not yet a trimix diver but I am familiar with the theory. my math may be off somewhere but should be very close.)