First of all some definitions.
The term purified water refers to water which has been produced by distillation, deionization or reverse osmosis. Simply stated any of the three methods can produce water of the same chemical purity for practical purposes.

I would say that if the choice was given to make a few gallpns of a 50 percent solution of antifreeze the 80 cents or so that a gallon of purified water would cost would be immaterial.

Finally you would not be adding iron, chlorine or any other substances in tap water to your brand new antifreeze solution.

Purified water has a known purity, tap water is all over the place depending on the location and even he time of the year. In the northeast, after the snow melts, lots of salt runs into the rivers that provide our water.

That's why I make my coffee with purified water or spring water, it just tastes better and is crystal clear.