it varies i think most places still offer a government rebate, i got a conversion in 2008 i got a $2000 rebate the system was $2500 the rebate has dropped off heaps now if theres one at all,
its not hard to set up legally it needs to be stamped by a licensed gas fitter, you could get a second hand system dirt cheap, what state are you in? ive probably got a system somewhere possibly on a panelvan im going to wreck cant remember but am sure its duel fuel, it costs $250 to have a tank restamped