One must also factor in the water used to produce and otherwise get the replacement filters to your door. The amount of water used per pitcher filter cartridge is likely much higher than you would expect, because you have to account for the water used in the original mining of the components, all the way up to the final rinse they get before they’re packaged.
Buying a long-lifespan countertop RO system will reduce your total water usage in the end. It will also generate less trash.
I installed an under counter RO system even though we’re renting because they’re so nice to have.