When I’m easy on the throttle I routinely see high 60s mpg figures on my ZZR-250. The absolute worst economy I had was 58mpg on a fill up. My personal best was 87mpg.
The ninja-250 will do freeway speeds all day, but in all honesty, the buzzing get to me in under an hour on the highway. So a 250mile trip for me would take at least three breaks to get off and walk about.
Since you are comfortable with big bikes, take a look at some of the slightly larger more recent engine designs. The 800cc BMWs supposedly get 60+ mpg (and that would be a very comfortable way to travel 250 miles).
Additionally, the 650 engine in the Versys (and thus Ninja 650) returns on average just shy of 60mpg accroding to webbikeworld.
TotalMotorcycle has a fuel economy comparison for a wise range of bike here
But it doesn’t seem to have been updated since 2006.
For sheer cash savings, a used Ninja is hard to beat (especially since it costs so much less than other bikes to buy in the first place)