Maybe 40 on a good day, downhill. It really, really depends on what's been done to/with your car and engine.
My 1275 -S gets about 22 MPG around town and since the aerodynamics are awful, maybe 25-28 MPG at about 60 MPH. The 998 engine will be more economical but again, any "go fast" parts added to your engine will likely lower the fuel economy.
When I see people quote 40 MPG and higher for a Mini, they are often in the U.K. The Imperial gallon is bigger than the U.S. gallon. Reduce U.K. MPG figures by 20% and you're close to the corresponding U.S. value. (i.e. 40 MPG U.K. = 32 MPG U.S. )