I can't really answer that for a number of reasons. The main one is that to gain the full effect from the co2, every other aspect of your growroom has to be dialed in perfectly. For example, if you don't have enough light, then the plants will not use up the extra co2 that you pump in. If the temperature is too high (even though you need higher temps to utilize the co2) or too low then the same thing. If your nute levels are too low the same.

So you can raise the levels of co2 in your room but if any other aspect is too low then the co2 is wasted. That is why a lot of people say to first get your room perfect then add co2 as a final piece of the puzzle.

Personally I can't justify the extra cost for my room either. I just run my ventilation 24/7.