I've looked into this too - the major issue that I can see is you need to either have the kit professionally installed or get it approved by an installer.
Some of the components (the tank as an example) are date stamped and are only considered 'safe' for a number of years - this means that you cannot just go and buy a secondhand kit off eBay and install it.
The main differences between the kits is in the way the gas is controlled, and it's the more complex controller that cost more.
I'd do some calcs and work out if it's worth doing ....from the tekki viewpoint it's probably a great talking point but from a return on investment view it may not add up.
Last thing to consider is that they need some regular maintenance, I've had a mate with a Range Rover blow the plugs out of the head and a mate with a focus have loads of issues ... both I suspect caused by a lack of correct servicing.
Cost is from about £700 fully installed, secondhand full kits on fleebay go for £50 to £200 ish
Edited by DomCr250, 26 February 2014 - 02:44 PM.