Cost is around $2000 for a conversion. You would have to plan to keep your vehicle for quite a while to make your money back on this. I don't think it adds any resale value to the vehicle. Best to go straight gas if you are going to convert, dual fuel is a compromise and you will lose more performance/mileage.
I had LPG (dual fuel ex taxi) on my Xf and it used to use around $25 worth of fuel a week (approx 350km). Car was a slug, motor was worn (over 300,000km) but I fitted a new motor and things didn't improve dramaticaly in the performance stakes.
You know, somebody actually complimented me on my driving today. They left a little note on the windscreen. It said, 'Parking Fine.'So that was nice.