Dude, ballast rebuild kit. You have to get the chord that plugs into the wall, and wire up a capacitor (should come with it). You might have to buy a socket to screw the bulb in, and you'll have to get creative with a hood, but I have about $110 altogether in my 400w MH, which is probably a little more than half of what it would've cost getting it at the grow store, and even cheaper than online.
Just buy an Hps bulb for that ballast (if its one of those ballasts that can run both) and find you a nice space for some extra flowering-- I'm sure you won't mind the extra
Edit: just to let you know, this is what your cost breakdown would be (in dollars):
Replacement ballast (for fixture): $70
Chord to plug in wall: $5-6
Socket for bulb: $7-10
Something for hood: found mine for $8