to be honest an RB20DET is probably not the most fuel economic engine to start with. Being a turbo engine it chews up ANY fuel, LPG or otherwise.
I could probably get a prius for like 14-15k and fark the ceffy off for 10k.
I've already owned an RB20DE GTS, which did 10k per litre, but it was gutless... so having owned one, I wouldn't wanna go back to one. Theoretically gas systems should be no less efficient than petrol systems. I've driven dual fuel systems, my gf had a dual fuel magna and that was such a pain to use... but I'm told gas only systems these days are sophisticated enough to work properly with no issues.
This is more an exercise in being different, and slightly thinking financially. After having spoken to Gas Research their waiting list is till May next year... but a little bird tells me they might do mine a lil sooner if I go with em