I assume your cost was .77 Euros... is your therm the same as an english therm or is there a metric therm?
If it is the same,
a therm is 100,000 btu
a gallon of gas has about 125,000 btu
125,000 / 3.785 = 33,025 btu
so 3 litres per therm.
By heat amount, switching to Nat gas look pretty good.