the other question to ask, do you live in WA? ... if you do you'll get another $1000 to go with the fed govt's $2000 subsidy, then that will total $3000 off the cost of you're conversion.
Mine cost me $1400 out of my pocket, it's almost a year old now (18k kms) - i'd guess it is almost or very close to paid back and either now or very soon i'll be in front financially.
There's a payback calculater here
LPG Autogas Australia
Also, the tank does rob you of bootspace by approx 1/3rd. There are other tanks, more or less rectangular shaped, that will be or are available that take up less room, these come at a price tho not that i have any idea of what that might be.
. : : Propane Performance Industries : : .
The installer that did my conversion has seen these tanks, but last time i saw him he didn't know what they cost. Hope they are affordable, i would have chosen these for my car at the time.