Now theres a couple of big questions!Cost wise, the main cost (i'm told) is in the Injectors - 1 per cylinder makes an 8 cylinder car much more expensive than others - typically you can expect to pay from £1800 to £2500 depending on the make and quality of the job - usually VAT on top as well.
There are a few companies that offer DIY kits for about 2/3rds of that which I thought of doing for a while, but then even though they check and certify it afterwards (you can't get insurance if its not certified) I still worried about the possibly options for my workmanship causing a large Disco shaped fireball!
The company I used in East Yorkshire is AutoGas Yorkshire who were absolutely fantastic - http://www.autogasyorkshire.com/ and tell them I sent you ;-) It was little over £2K+vat. They even lent me an LPG DII while the work was being done (takes 5 days) to get used to it.
Halfway through I noticed that the one they lent me had 2 LPG fillers - one on each side - as you tend to find that most smaller stations only have one LPG pump with outlets either side, and theres always some guy filling up a van on the adjacent pump! With 2 fillers you can park either side and it makes life MUCH easier. I rang them up halfway through the conversion and they said no problem to add, and only cost £90 - told you they were good :D
Cost of fuel wise - I did hear that they had promised to keep the duty low until 2009 though I don't know if thats still the case. I would expect the price to rise slightly, but not by enough to make it non-economic in the near future. Basically do it now before they notice!
On the plus side, HM Revenue has just announced that small producers of BioFuels (pretty much home convertors) can now pay virtually no tax on doing it all which is a massive change. They also claim that they will review this over the next year or so to include larger convertors. This would suggest that the "green" fuels are currently in favour so you never know - it might get better for LPG too in the not too distant future.
One sobering thought though - in general, LPG is a by-product of the petroleum refinery business and without Petrol being refined, there would be no Liquified Petroleum Gas. Therefore if more than 50% of people switch to LPG, there won't be enough petrol being made to make the LPG ;-):P:w00t: