Are you mechanically inclined? Do you like to tinker? Willing to learn?
You can pick up an older bike for next to nothing sometimes. An older CB, KZ, XS or something like that. Charging systems components on some of the older bikes can be pricey, but if you can buy an older bike running for $500 or so...well, that leaves cash for mods.
If you are wanting to go "streetfighter" don't get stuck in the mindset of needing a modern sportbike to pull that off. There's lots of '70's models bike that can be had for cheap that make awesome fighters when done. Go look at the current FOTM & FOTY threads for some really good examples.