RO water. I don't know where you're getting water analysis done for $20 a pop (please share with the rest of us), but if it changes that often, you have no idea in between analyses what you have anyway.
The water additions are not all that expensive, and they last a long time. Further, there are water spreadsheets that help you figure out what to add to the water, so you don't have to be a chemist to figure it out. Brunwater and EZWater are just two of them.
My last brew had a little gypsum (CaSO4), some calcium chloride (CaCL2), some epsom salt (MgSO4), and 3 ml of Lactic Acid. Generally it's about 6 grams or less of each, in some cases as with gypsum maybe just a gram or two. This for 8 gallons strike water, the calculators let you figure this all out.
From Ritebrew, 1 pound of Calcium Chloride is $2.69; 1 pound of Epsom Salts is $2.59; 1 pound of Gypsum is $1.59.
A 2-ounce bottle of Lactic Acid (88%) is $1.89. You can get
plastic pipettes from Amazon or maybe your local farm store if you have one--or even an eyedropper if it's graduated in milliliters.
You might also want some pickling lime, baking soda, and (maybe) chalk to raise the pH; I've never needed to, but I don't brew stouts and such dark beers that require it.
So--you can get the necessary salts and such for the $20 you'd spend on an analysis. All you need then is RO water, which you can buy. Or you can get an RO system to make your own, which is not only cheaper but more convenient than schlepping 7 or 8 gallons of RO water from the store every time you brew.
I have sink in my garage; I bought an RO filter from Buckeye Hydro, connected it to the faucet with a hose adapter, and voila! RO water. I emptied it into a 7-gallon aquatainer and that's my source of RO water every time I brew.
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I've since permanently installed the filter (which you see hanging on the wall) underneath the sink, and installed a line to feed it so I don't tie up the faucet. Originally I wanted to be able to take that filter in the house if the garage dropped to freezing temps, so it's portable. You could also mount it on a wooden stand and store it in a closet between uses.
Takes about 4-5 hours to get 7 gallons, depends on how warm the water is--warmer works faster. I fill my kettle with RO water then set it to filling again.