Aerial refuelling – what does it cost?

I’m curious about something. It is the cost if air to air refuelling. Let’s say jet fuel costs $3.00 a gallon delivered to a fighter base and £3.00 a gallon delivered to a tanker base. Is there a ball park figure for the cost of refuelling a fighter 40,000ft above the tanker’s airbase (0nm range from the tanker base)? Same question for refuelling a fighter 1,000nm from the tanker base.

I’m trying to find out how much more cost is added for delivering fuel in the air rather than on the ground. Disregarding the cost of building the base (it might have already been built for some other use before it was used by a tanker squadron), how does the cost of buying tankers and operating them translate into additional cost above the $3.00 a gallon that I mentioned? If air forces have budgets (which contrary to quite a few posters’ ideas, they actually do), this should have a bearing on the range required of combat aircraft.

So are we talking of fuel delivered in the air costing 5 times as much 40,000ft above the tanker base? 20 times as much 1,000nm from the tanker base? Can anyone come up with some ball park figures?