I saw an interview with a brewmaster and he said whether a beer is made of spring water or anything else is kind of BS because they can make water with whatever properties they want, it's not rocket science.
But I personally know a beer brewer that was "remineralizing" water to a certain specification, and at some point they stopped because it was far too expensive. So I'm not so sure about the claim that people can make water with whatever properties they want, at least not in a way that is broadly economically viable.
I'm guessing that the beer brewer you spoke with was talking about the cost of buying distilled or RO water, as opposed to the cost of the water adjustment itself. It's probably a lot more economical if you're cleaning and reusing graywater, vs. trucking in distilled water, or running municipal water through an RO filter and essentially paying twice for water treatment.
it's a mixture of gypsum, calcium carbonate and epsom salt, conveniently pre-packaged as Burton salts.
I use 1/4tsp per gallon of distilled water. the expensive part is the distilled water (even more-so than the co2 used to carbonate it).