No solid numbers, but WAY less. My initial calculations/estimates were something like this: Run the fan for 2 weeks straight 24-7, on the amount of power the A/C uses in one day. Or something.
Too many variables for me to pin something down but our peak August bill went from $178 to 147. The rest of the year is high 40's to 80's.
July and August are worst case scenario because you may not see evening temps below what you want to start the house off in the morning.
The significant savings is the other warm months of the year that you might run the AC for 2 hours a day opposed to 8-14.
The side benefits of purging the house while cooking, painting, working in the attic...whatever, we run the fan a few minutes every day even during the winter to keep the air fresh.
It's also nice not having floor/box fans staged all over.