I'm not sure what the price is in the UK but in the US you can get a system with tank for about 7000. Now that's the top end with all the EPA stuff. An old fogger or aspriated system can run about 500. But with the ECM and new fuel injection systems you can have problems backfiring and blowing apart your intake and manifold with the fogger.
If you are able to play with the ECM you can get just as good or better mpg and performance. Timing, compression and even the spark plugs make a difference. A good rule of thumb with propane is "Lean is Mean."
CNG or compressed natural gas would be nice, BUT they cost lots more and not as efficent. Not putting them down cause I work on them too. Just not my first choice.