If you're just starting out, ask around at the track, there's nearly always someone who's got a frame, if not a complete bike, they're looking to move.
Additionally, your first few sessions would be well-spent on using the track's loaner bikes. Bring your own pedals and they'll get you all set up. Among the benefits of the rentals:
- you get to dial in your fit without spending any money.
- you get to ride your first couple of sessions on the track without worrying about falling and trashing your bike.
- you get to find out if this is something you really want to do. Track racing, like every other discipline is not for everyone and if you spend the money on a bike, then realize you prefer road or MTB or CX or whatever; or even if you just don't like your local track's community, you won't have wasted your money on a bike you'll never ride.