I am led to believe it is very similar MPG (someone might like to confirm this) but at the moment and for the forseeable future the LPG will remain at half the price of petrol.
The government offer incentives and might help to fund the cost. I believe not all cars are able to be converted.
Here is details of my car with a conversion quote including savings and investments
Vehicle: Honda Civic 1.6 SR Coupe 2d 1590cc
LPG System Greenfuel (20314)
Cost of Conversion £1,730.00 + VAT
Net Cost of Conversion £2,032.75 inc VAT
Petrol Spend PA £1,483.54 per year
LPG Spend PA £814.51 per year
Saving PA £669.03 per year
Return on Investment 33%
CO2 Saving 0.76 Tonnes
Approved by top 20
insurance companies Yes
This is based on my car doing 12,000 miles a year. I would need to own my car for 3 years before i got any benefit.
See info here: http://www.greenfuel.org.uk/