That’s for you to decide. But the environmental impact of batteries is drastically reduced if you actually recycle them and so forth.
As for cost effectiveness?
Quick google of a poorly documented site that looks “about right” says the average price of a gallon of gasoline was 7.079 Australian dollars and 4.459 USD, respectively. Normalizing, that is 4.54 and 4.459 USD. And it gets a lot higher in other countries, but those seem the two pertinent to this discussion. www.globalpetrolprices.com/…/gasoline_prices/
So doing a bit of math:
X city miles / N city Miles Per Gallon * 4.5 USD per gallon = M dollars per retrofit
So get the price of the retrofit, your average city (stop and go) miles per gallon, and figure out if you are likely to travel that distance. Because the effectiveness of a hybrid drops drastically for extended highway (although 100 kilometers is a decent charge size), whereas they use trace amounts of gas during stop and go city driving.
But also, like with anything, it is a bit of a gamble. Do you think gas prices will go up or down? Do you expect to continue driving your current route? And so forth.
Also, that rough equation assumes inflation will increase (because it always will) and attributes cost of charging at night as well as lack of perfect efficiency in regenerative braking to that.