I have a 40g setup with 20g sump and will be doing 20% WC per week which is about 8GPW since neither tank actually has that much water in it.
At 8 gallons per week, plus top off water, I use around 10GPW total. That's two buckets worth. It's also only $5 a week if getting it from my LFS.
If I don't want to lug around buckets every week, they sell RO/DI space saver units at 75gpg on eBay for $60 with good USA filters in them so that I can do it at home.
That's $60 upfront, and then at 50 cents per gallon the LFS charges, after 120 gallons made, it pays off.
Now onto the price of water. It produces one gallon pure for every 2-3 gallons waste. Sound like a lot at first to some.
But when I need 10 gallons a week, it "wastes" 20-30 gallons of water which goes into water containers outside for plants outdoors and indoors (takes no added time to fill them as I don't even have to be home to do it)
Water rate is $3.45 per 100 cubic feet of water.
Sewer rate is $4.70 per 100 cubic feet of water used.
How many gallons are there is a 100 cubic feet?
There are 748 gallons in 100 cubic feet.
Which works out to .005 cents a gallon.
Multiplied by the 40 gallons a week it uses, and you get 20 cents.
And keep in mind, saving all the water that comes out, you eliminate your sewer rate.
Once you actually crunch out the numbers, it's pretty amazing.
I can honestly say that not even I was aware of how inexpensive water really is. I always thought I was paying more than that.