You aren't going to find a bike that goes a lot cheaper than a 250 Ninja. New ones can be found out there for under $4,000 and older used ones show up as cheap at $700 in my local Craigslist. Your best bet is to go on ebay and check the closed auction prices on small bikes. That will tell you what falls in your price range.
There are lots of good choices, but if you don't think you can afford the Ninja you want, I think you will be really happy when you start to do your research.