It depends on which year of the car you want. A 2005 BMW Z4 Roadster gets 27 miles per gallon on the highway, and 18 miles per gallon in the city. A car that is more recent might have better fuel economy.
The base price for a new 2011 BMW Z4 Roadster is about sixty thousand dollars. When you add in desired options, the cost can exceed one hundred thousand dollars. However, you could find a used one...