Hey QWK-1E,
I've got a standard 5L VR Calais and I get around 400 to 450km a tank. That's average stop/start driving. Put your boot into it and it can get down to 300km, take it easy and I've had 500km out of it before. On long runs they are great, I've had 600 to 650km on a tank before, but I've heard that 700km is not uncommon. My V8 costs me about 5 bucks more a week on fuel than my missus' V6 VS, at about the same km's. She's a bit of a lead foot though (she's the one who got 300km out of the V8).
I may be wrong but I thought the 5L sedans were supposed to have an 80 litre tank but I can only get 60 in mine.
Filling it up depends on the price of petrol. Times the capacity of your tank by the price per litre of fuel.