There are great deals out there on good used bikes. You should wait 'till you're out of school and working and making some money to but a new bike. After all, if it's not needed for transportation, it's just a toy. And you will need that money for other more important things now and later in your life. You are trying to rationalize a bad decision - getting into debt or spending money now for instant gratification and pleasure and it's not worth it, look around and get a good used bike for 1/2-1/3 the cost if that's what you really want.
Why do you want a new bike? Why don't you keep the Ninja for the foreseeable future? And do you have a car? You can't ride the bike where you live all year to go to school or anywhere else.