Basically, try to break each fuel source into cost per kwh and then add in efficiency of heat converter (boiler).
Natural Gas: approx 6.5 cents per kwh (extra saving from benefit of modulating boiler as it will modulate flame to suit demand, i.e. turns flame to low flame when system gets hot or if system is zoned and only one zone is on)
Boiler efficiency (Seasonal approx 90 - 91% / Combustion efficiency approx upto 99%)
LPG (own storage tank): approx 9.1 cents per kwh (same benefit as Nat Gas boiler)
Boiler efficiency (Seasonal approx 90 - 91% / Combustion efficiency approx upto 99%)
Kerosene: approx 9.5 cents per kwh (no modulation so boiler will produce more kw than is required & wastes fuel)
Boiler efficiency (Seasonal approx 93 - 97% / Combustion efficiency approx upto 99%)
Electricity: approx 20 cents per kwh (100% efficient)
Solid fuel is very difficult to assess, due to moisture content, varying quality, varying wood types, etc.