Not trying to be a douche, or stray you from getting an answer.. I can't answer you the price, but I can say this. It's a Ninja 250. They're extremely cheap brand new--nauseatingly cheap to get a hold of used. The cost of rebuilding a 250 motor and putting it back in? I'd say it would vastly outweigh what the bike would be worth. I'd just buy another 250.... Or upgrade to something else.
Might could get away with pistons, rings a a bore job. Valves?? Well I'll let you know when I do one if I keep the bike. It has almost 5K and has had the piss run out of it at the track. Keep an eye on the oil bruning and compression.
The engines are kinda disposeable, you can get one on ebay for like 100-200.00. Not really that expensive. Most of the time the engine gets to that high of mileage it has been owned by about 5 or 6 different ppl, so the mileage may vary on how hard it was ridden, how well it was taken care of etc. From what I understand they are simple to replace if this issue is what is concerning you about buying a 250, dont worry about it you will probably be looking for something b***** before you reach the switch the motor phase. But if you do like the bike that much to keep it you will be able to switch the motor out yourself for little dinero.