How much does go-around cost?
We can look at financial data from airlines to estimate these figures.

In addition to fuel, there will also the extra duty time for the crew, some delay in schedule, and extra flight time on the aircraft. The delay in schedule can be difficult to estimate, but other costs of operation can be estimated. A short flight would take 1.5 hours, and the go around might take 0.2 hours. 

Based on the financial data, an MD-83 might cost about 6000 dollars/hour to operate. This includes all direct operating expenses. This means that the regular flight would cost about 9000 dollars to operate, and a go around would cost about 1200 dollars. Since a go around uses a lot of fuel, the figure may be a bit higher.

The figures for a Boeing 747-400 show about 15000 dollars/hour to operate, and a Boeing 777-200 about 12000 dollars/hour to operate. That would make an 8 hour flight 120,000 dollars and 96,000 dollars respectively, with go arounds costing 3000 and 2400 dollars respectively. Obviously, for longer flights, the go around will cost less proportionally than for a shorter flight.

As DeltaLima mentioned, the cost of not doing a go around (which includes making this decision early enough) can be substantially higher. Loss of or even just damage to an aircraft will cost much more time and money.

To see if these numbers make sense, this report mentions that an empty MD-83 would need 1000 kg of fuel for a go around and new approach. It also mentions that a ferry flight from Paris Orly to Olbia, Italy, would normally require 6800 kg including reserves. If reserves are about 1600 kg, this would mean that the flight would use 5200 kg of fuel.

If jet fuel costs 3 dollars per gallon, this comes out to 985.72 dollars of fuel for a go around, and 5125.73 dollars of fuel for a regular flight. This figure seems reasonable along with the figures above.