LPG consumption is in practice approximately 20% higher than petrol consumption on the same vehicle applying the same driving caracteristics.  To work out the cost effectiveness the following assumptions have to be answered:
1.  The annual distance to be travelled (Km)
2.  The vehicles fuel consumption (L/100 Km)
3.  The average petrol cost ($)
4.  The average LPG cost ($)

To estimate the petrol consumption cost the following calculate should be used:Km / x L/100 x $(Total distance travelled in Km x Litres of petrol/per one hundred Km, x the cost of Petrol per Litre = total annual cost.

A similar calculation applies for LPG, except that fuel consumption will increase by (approximately 20%) so the fuel consumption figure must be increased by factor of 1.20 and the LPG is substituted for the petrol cost Km/x L/100x1.20x$

Per country LPG specific conditions might apply such as grants/subsidies when converting your vehicle. Check your local/central governments schemes.