I had no question that it would eventually pay for itself, but as your figures suggest, it would take 2 years or so (minus your time and effort for refills). So in terms of $ spent on your setup, I think the extra CO2 from running it 24/7 is pretty negligible.
The other incalculable "cost" is - I don't know - resistance by the technologically inept?
Making the jump to pressurize can intimidate some folk. Eliminating the solenoid/timer/ph controller can make it a little more understandable.
Along the same lines, just lighting, filtration, and heater generate several plugs/and a timer or two - quite a nest of snakes. Adding another couple may require repositioning the powerstrip or somesuch.
Final variable - different folk tend towards different ends of the hightech - lowtech continuum. Doesn't make either better or worse than the other.
No question where I lie on that continuum - I bought my set up used, and immediately turned around and resold the ph probe!
And I assume the guy who bought it from me is happy as well!