How much it will cost to fix a broken gas gauge depends on if you fix it yourself or if you have to pay labor cost. It also depends on the cost of the material to fix the broken gas gauge. That said, as of 2013, it can cost between $100 - $200 to get the gas gauge fixed.
1 Additional Answer
How much it costs to fix a broken gas gauge depends on what part of it is broken. There are several thing that can go wrong with it including the sensor in the actual gas tank. These all have different solutions and different costs associated with it.