to really answer that question I'd have to really try to replicate the same conditions with it on and off, its really hard to determine cause each day the usage of the phone varies from one extreme to another. If I were to guess it would probably save about 2% to 7% (sync off) just from the varies apps that want to poll the internet ie the weather widget etc. from locations of good 3g to areas of bad 3g over a few days (depends on what widgets you have running that need 3g). I have tested the battery with it off and very little phone usage, I've gotten it to run 167 hrs till 35% battery left.
Edit further testing done.
3G Data turned off phone light usage battery 93% at 38 hrs 10 mins
3G Data left on " " 93% at 12 hrs 31 mins
just from this unscientific test, battery life with data on is reduced by approx. 2/3