Ok - assuming 40p/l for gas, 82p/l for unleaded and 30mpg when running on gas, 35 on unleaded these are the figures: LPG = 6p/mile. Unleaded = 10.6p/mile. Saving on LPG is therefore 4.6p/mile.
You can buy a DIY kit for as little as £500 - payback mileage is 10,869
A more expensive DIY kit at £1000 (or a budget fitted kit) - payback mileage is 21,739
Top notch fitted kit (sequential injection) will give better fuel consumption and a saving of around 5.5p/mile. With a fitted cost of £1500-2000 the payback mileage is then payback mileage comes out at 27-36,000 miles.
If you can find gas cheaper than 40p/litre (and it is relatively easy to find it cheaper) then the payback mileage comes down.
I paid £1100 for a good quality kit for my Voyager. I fitted it myself and with fuel consumption of 23 mpg petrol and 20mpg gas I calculated a payback mileage of about 12-13,000. It was relatively low mileage because my car is less economical (giving great p/mile saving) and I can buy gas through "Countrywide" stores at about 33p/litre.
Cost per mile, my Voyager gives me the equivalnet of 45mg on unleaded. Want to buy it from me?