I'm under the impression that the usage is more like 20% bacause it has about 20% less energy pre unit than gasoline. I suppose if you just want to be a lead foot then you'll use 40% more to get back to the same power output

so when gasoline is more than 40% more expensive than propane it becomes cost effective. If there ever is hydrogen or methane or whatever then you probably won't need to change much to use that fuel.

Since the there may be less taxes, you could get a 10 - 20% discount off the top, so that means gasoline only needs to be 1/4 more. TAX FREE IN AU till 2011!

I was looking and for example in the UK:
unleaded 95: 87.2 / l
diesel : 91.1 / l
super : 89.5 / l
LPG: : 34.0 / l

95 octane is 2.56 times more than LPG. this is also because LPG doesn't have 66 - 71% in taxes added to it.
source: http://www.theaa.com/allaboutcars/fuel/

A LPG kit in the uk costs about 350 for a datsun.

all the parts for an LPG kit don't weigh 50 lbs. ( when tank is empty )

something to consider...