My kids beg me to make this all the time, not just halloween!!!
I use 3 cups of sugar and 3 - 2 litre bottles of Faygo Twist (OR Sierra Mist) for a total of 6 (six) litres of liquid.
Faygo is my 1st choice only available in Michigan, Indiana and Ohio, I think. Sierra Mist is my 2nd choice.