How much money for gas in camper van from melbourne to darwin?

As the distance between fill ups is excessive in places, it is advisable to fill up, and then carry 2 or 3 twenty litre jerry cans with you also.
Allow $200 - $300 for fuel.
If you are relying on your vehicle to get you all the way north....
Check that the oil is topped up.
Check the radiator water is topped up.
Check break fluid is topped up.
Do this check every time you stop for fuel.
Bring 40 liters of water as a back up (for you and the vehicle)
First aid kit.
Food and liquid.
Buy beer at the stop points.
Dont ever underestimate the outback, when it bites you you stay bitten
Have a great time

I think you should budget for about $200 one way if you are going to South Australia and straight up from there. As soon as you hit Alice Springs the price of fuel goes up. Also, take some spare petrol containers (3 is best) full of petrol take a funnel to put the fuel in the tank as well. Oh by the way make sure you have cash. Loads of petrol stations on route have no card facilities. Please make sure that you have completed all of your planning and have all the spare parts and survival equipment you need.

Good question, How long is a piece of string? There are so many factors to take into consideration it would be almost impossible to guess

It depends which way you are going but it is going to be about 5000km. A camper van would be lucky to get 10litres to the 100km, more like 15litres. At 10km you will need 500litres of petrol; at 15 litres, 750 litres.

Petrol prices vary but you might be lucky and average $1.30 a litre. That means that at 10litres per 100km it will cost you $650 and at 15litres $975. That is what I would budget for the trip. If you get better fuel consumption or find cheaper petrol you get a bonus.

good advice above. one more thing. book a campervan powered by diesel. do not think of gas/petrol as it will double your fuel bill. Avoid roadhouses and servos on the way in or out of towns for fuel if at all possible. they are burglars.look around the towns for a fuel depot,ask a local where they buy their fuel.save coles and woolworths supermarket dockets for discounted fuel.