Generally costs about £1500-£2000, depending on the car. LPG is slightly less efficient than petrol, which would mean an increase of 10-15% in your fuel consumption, although LPG costs half as much as petrol, and the power would drop by a similar amount, but unless you absolutely rag it everywhere, you'll hardly notice it.If you plan on keeping your car for a few years, or do a huge mileage, it's worth it. Dunno if you'll have much boot space left when the tank goes in though!