The low end Bianchi Pista seems to be available around here (LA) for $500 or less (a lot of people seem to have gotten them cheap through shop discounts). It's heavy but it works fine. I think the bolt circle for the chainrings is something odd (as in not 144), so you won't be able to borrow chainrings, and some common track sizes may be hard to come by.
As Dial Tone said, Ebay is a good bet, or some other used source. If you have a little patience you can probably get a decent deal on a used one with good components.
And you can't use bullhorns in mass start events on the track, though they're ok for time trials (usually paired with aero bars)