For a 250cc motorcycle I suggest this...
Oh you meant cruiser.
A good city bike I had was my first bike. A 1994 Yamaha SecaII. It was a 600cc Air Cooled motor (Though I didn't feel I had problems from overheating in bumper to bumper). It got 45-50mpg city (With me being new and floring it half the time) and 70mg+ highway. It had great midpower for cruising around 4-6k and needing to get out of the way quick. Also a super awesome comfortable seat and a comfortable stance. I paid $1500 for one in great shape but have seen them go for $1000.
Also would you consider a scooter? I know a guy with a Aprilia and a Harley who rides a Kymco scooter everday. I think 150cc with 68mpg city and he paid $1500 for it brand new.