Adjusting water chemistry is extremely prevalent in brewing. For example, if you've ever had a hazy IPA, part of the softer bitterness comes from high levels of chloride in the water. The cost of common brewing salts (gypsum, calcium chloride, etc) is a small fraction of a penny per beer.

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.