You can buy a used ninja 250 for around a thousand dollars.. If you want to buy a new one for cheap you can get a ninja 650r for about $7,000. You can get a used 636 in good shape for around $4,000.
Do not waste your money on a 650R. It is over priced and under powered. I would get the 500 before the 650R, the performance to cost is much better. But for about $2500 you could get a used 250. They sell new for about $3000-$3300.