I use 200 gallons of RO water and another 400 gallons for the mister and swamp cooler of tap water a month. I also grow orchids.
150 to 200 gallons ,
i collect rain water. into two 55 gallon drums.
During the growing season, I buy 2-4 gallons of distilled water a week at about $1.09/each. That comes out to $17.44 per month. I'll round up to $20 a month. Sometimes I have to buy extra if I'm doing lots of transplanting.
That is for about 8-9 months out of the year. In winter, I'll buy maybe 1 gallon a month.
It will probably be more this year though, as I am keep some tropicals in a mini greenhouse over winter.