It depends on how many miles you plan to do in the car. LPG conversions cost about £1200 for a four-cylinder petrol engine (but can rise to £2000+). If you're doing 10,000 miles a year, you'll save roughly £600. So you'll need to clock up at least 20,000 miles in the car before you break even on a £1200 conversion. You also won't get the price of the conversion back if you sell the car. You'll also lose a bit of boot space - although that's not too much of a problem with this model as the boot is huge.