I’m not sure of you water usage and what you intend to use it for. According to their website, the lifespan of the filter is for approx. 50L of water for hard water with TDS up to 300ppm.
If you plan to use for your coffee machine exclusively, also bear in mind on the frequency of the filter change. For Brita Jugs, they advise changing the filter every month at least due to bacterial build up. I tried to look up what the frequency should be for the ZeroWater but they don’t say.
I use distilled water. For my usage for the coffee machine, approx. 1.5L per week, the cost of running the unit is approx. £20 per year. This is already factoring in the trend of ever increasing energy prices.
If I had to buy a ZeroWater, the cost of the filters alone would exceed the cost of running the distiller (£40 x £20 respective). If I had to change those filters every month, I’d spend £240 per annum.