the fuel average depends upon a few things some of which f_alik already mentioned...
engine and carb condition...if they are in good order a 250 should give between 22-25...
ur throttle control...if you have a habbit of reving the bike even when you are standing still(like me :P)...it will decrease the mileage..
traffic condtions...play a great role..
and if you are rolling on 40-50km/h in 3rd or 4th gear...it will increase the average slightly...but then again wats the point of having a sports bike then