We had the same concern regarding plastic bottles for sparkling water, and tried sidastream, which worked fine for awhile.
I didn’t do a detailed breakdown of cost, but we stopped using it, and returned to buying a 6x2l pack locally, and recycling the bottles.
Whilst sparkling water might not be for many, we have foregone any alcohol since 2011, and with our other dietary choices believe we have reduced our environmental cost in any case.
You have to have some pleasure in life, and sparkling water is ours!