The price of a liter of gasoline today in Berlin is about €1.462. I'll save you from the math: that translates to about $7.47 per gallon. You can always find up-to-the-hour prices for gasoline and diesel fuel at Gasoline-Germany.com.
You can expect to get about 35 miles per gallon in a compact rental car in Germany. Thus if you average 100 miles per day on a 14-day vacation you can expect to use 40 gallons of gas at a total cost of about $299.
