I don't have the figures in front of me, but propane, which is metered and sold by the gallon, is much less dense than heating oil. Like 40% less. So that 500 gal/month is roughly equal to 300 gal/mo of heating oil. Which sounds about right for an older home.
On an energy-per-dollar basis, propane is a lousy deal! (We use it for dryer + hot water).