ninjettes are absolutely everywhere. super abundant, very well documented (ninja250.org forum), cheap parts that are easy to come by, easy maintenance. there is a reason the model didn't receive much in the way of changes over more than the decade it was out. if it ain't broke...
they are stupid cheap as a standard, but since the 300 came out like it was said above, they are only getting cheaper. also. they are fun! not too many bikes you can rag on full throttle. bigger bikes would have you on the ground wondering what happened. the ninja is very forgiving. the quintessential beginner's bike, while still being extremely fun for the seasoned rider looking to push the limits. it also has a huge following in production races, road racing, and commuting. it's an absolute steal what you get at the cost. i see them all the time in the 1200 to 1600 range. another awesome point is that they hold their value well. they have been at that price for the last 8 years or so. if you need to sell or decide that riding isn't for you, you can typically get your money back - sometimes even more.