Even on 2G only, I think the drain can be severe in an area with very poor signal, because the phone uses much more energy than normal trying to contact a mast.
If you look at Settings > About phone > Battery > Battery use, you would normally expect the display to be the greatest drain on the battery; but in an area with very bad (almost non-existent) signal, you might expect to see Cell standby at over 50%.
In the building where I work, almost no radio signal gets through, so most of us here find our phones reporting "Emergency calls only" and prompting to choose a network.
But even there, I'm only losing about 25% in six hours. The phone gets quite warm though: about 38 degrees C - in airplane mode, it's down to 33 degrees.
I've stopped most things from syncing too much though. Do you perhaps have some apps synchronizing very often (and which therefore do very little when the radio is off) ?