For starters it depends on how clean of a job you want - the cleaner and more professional = $$$ For example I build wiring looms for pro touring cars starting at 10k but in the case of this GU and his budget I did all the wiring for 1k. What engine you choose and condition will cost you $$ aswell. As far as parts needed for the conversion if your coming from a 3l you could get all the parts needed for under 5k if your handy and make your own fuel systems etc. OR the full marks kit is about 8-9K. Standard patrol radiator is more then ample! As for how I get 11-13L/100 around town and 9-10L/100 on the highway Is a few in house secrets but you will get pretty close with matching your tyres & diff + the correct tune. I own a 2002 Chev tahoe with a ls1 (heavier 4wd then the patrol) and i get 11L/100k
If your doing it all yourself and not afraid to make things then you could do the whole conversion for less then 10k easy. If your paying a shop it could be anywhere from 12k upwards.