I don't think it was the same price but I know it wasn't twice the price. Your probably supposed to use distilled water but ive used tap water every time and never had any issues.
Found this while trying to collect my thoughts on the 100% coolant.
It is not recommended to use 100%, but anywhere between a 50/50 mix and a 30/70 (30 being water, 70 being the antifreeze) mix is good. if you live in the south, 50/50 is the most you would want, but if you live in the north, 30/70 is a good idea. most people answering have been saying that pure antifreeze will freeze sooner than 50/50, but in fact, it has by far the most protection from freezing, if that were all you cared about, then that would be great! What you need the water for is to move the heat around, ethylene glycol (the main part of most antifreeze) is a very poor conductor of heat. also, the reaction that at least one answer talks about is for the boiling point, not the freezing point. the peak freezing point, as i said earlier is with 100% antifreeze, however the peak boiling point is with 67% antifreeze. the final reason for the mix is corrosion, and the best mix to prevent corrosion is generally with a 50/50 mix.
As most have said, i would recommend that you get the right mix in, but it is not the end of the world if you can't fix it, just use the right mix next time.