GH pH down is phosphoric acid. Using a cheaper source of that would be ideal.

The price for GHs dry pH down product is the same as a quart of liquid. It is equivalent to more than a quart, but I don't know how much.

Though I wouldn't hesitate to try sulphuric acid if it is at least fertilizer grade.

I occasionally use 1% Sulphuric acid, obtained from the dilution of 33% battery acid. But - most of the time - I use 5% white spirit vinegar.

If your pH is continously going up, then your nutrient stock may be dirty. I recently bought a fishtank canister filter, and put it in the nutrient container for a few days. It clears the nutrient and stabilizes the pH.