ZX2r is an awesome bike, price can never decide the condition of the bike, rather it is the condition of the engine itself that should decide about it. But as far my little knowledge is, CBR and FZR have been above ZX2rs and GSXRs in the 250cc class, though foosa had every right to disagree.
Mileage, again, depends on the condition of the bike. I have got just under 17km/liter on my FZR on normal rpm drive, and I am optimist that with a round tyre on long ride it had the potential to give me upto 20km/liter. There is no limit for lower average. If you drive it the way a sport bike is driven then you'd be happy to get 10km/liters.