Cheapest original equipment replacement antenna I found was $40 US plus shipping. The internal helix antenna is a very accurately cut and sized piece of foil with an active amplifier circuit inside the plastic case.
Due to the weak signals emitted from GPS you need the best antenna possible to ensure the system works reliably. After market patch antennas are cheaper and work well so long as they have a clear view of the sky.
Unless you can find a decent undamaged one second hand, you are going to have to bite the bullet and pay up for a new one.
Pick a GPS specialist site for cheaper prices rather than aviation sites as their prices tend to be a lot higher.