1) The CO2 savings are nothing more than a wild guess rather than a scientifically sound estimation. The real numbers might be as low as 600000 tonns. And even these numbers are unsure because they include all those car drivers who have used this ticket only once during the 3-month test period, out of curiosity, and therefore can't be used for reliable prediction of what would happen if this kind of tickets will be introduced permanently.
2) At the same time, we don't know the real costs of this experiment yet. Some sources call it to be 10 billions instead of 2,5 billions.
3) Overall, only 10% of ticket users chose public transport instead of their car. Most of the trips has been generated by people who wouldn't travel at all if the ticket was not so cheap. And on the other hand, there are many news that people who usually commute using public transportation had to fall back to the cars, because the trains were so overcrowded and were frequently cancelled.
More scientific studies are needed to research the REAL effects of this experiment. We shouldn't make any political decisions based on the wishful thinking only.