Apparently, the first answer to this question has no idea about bicycles!
A good, inexpensive TRIathlon bike or light road bike might be something like an older model (bike stores in my area still carry them) Trek Equinox 5 - it's a year or two old, but it's all aluminum and comes with aero bars and good components - light, but heavy by Tri bike standards.
A very light tri bike is going to run you closer to the $3K-$5K range (carbon everything, bar end shifters on the aeros, carbon wheels, etc.).
If you're looking for entry-level tri bikes, and you've never run a tri before - just trying it out - my suggestion would be to use whatever bike you have to train, and rent a good, high-end tri bike from a local bike shop to race with. Some shops in my area allow you to rent high-end Quintana Roos - nice, inexpensive tri bikes - which, by the way, might be your answer, if you can find one in a shop or online that is within a few years old.
Here's a good list:
High End:
Trek Equinox TTX (about 3500)
Quintana Roo (1000 - 5000, depending on the model)
Cervelo (very pricy - can be up to $5K)
A good beginner would be to purchase an inexpensive road bike - say around $800 to $1000 - a little smaller than what you'd normally ride (for instance, I ride a 52 cm bike, but for a tri, I'd ride a 50 cm bike) - and then purchase some Profile aero bars and add them on. If you don't like Tri-ing, then at least you have an inexpensive bike to either sell or ride in road racing (which is fun).
Good luck!