Go check out Fuelly.
http://www.fuelly.com/motorcycle/kawasaki/ex250
Seems that the mid-50's (US) is average mpg, with some of the best in the 70's and some of the worst in the 40's. I've seen posts on this site from people that are
really into fuel economy and they get AMAZING numbers.
Personally, I get around 60-65mpg. It doesn't seem to matter how I ride in terms of hard or gentle, but it makes a big difference dependent on load. If I have tank bag and panniers on, the fuel consumption drops quickly. If I carry a passenger, it drops further still. I don't think that the little engine is intended to haul around a lot of drag.