Assuming no external losses (fluid to air), you will need to remove
roughly 5,925 Btu's from the wort (8.345 lbs/gallon * 5 gallons * 142F temperature drop) to get from boiling to 70F. This is conservative since the thermal capacity of wort is higher than water given the presence of dissolved solids such as sugar.
Assuming your cooling water starts at 32F, you would need 3.89 times the volume of cooling water (142/38) to get to 70F as the initial wort volume, or roughly 19.5 gallons. This assumes perfect heat transfer, and again, no gains or losses from convection. Adding ice from the freezer may lower the volume required since this ice is at 20F or so not 32F, but the volumetric requirements are still very high for a closed loop system. I'm curious, how big is the rubbermaid container you are using for cooling water?
I think the question that should be asked is what is more important to conserve, water or electricity? I think the answer will depend on where you live and which resource is more scare. Chilling water to make ice in the freezer is very energy intensive... especially when you consider that 75% of the energy in your electricity was wasted just getting the electrons to your house. Power generation also uses a tremendous amount of water for cooling (that's why they are always located on lakes or rivers). I don't have the time to run the math, but using electricity to freeze water isn't going to save a lot of water in the grand scheme of things... it may even use more overall.
Considering my tap water is 45F in the winter (and that water was chilled by the cold environment, not electricity), I'm comfortable using tap water to chill with. Besides, in the winter I use the first several gallons of hot run off to wash all my brew equip., and in the summer, I use it to water my lawn.