Check on ebay and you should be able to get a Garmin 18X for as little as about $35 if you keep watching.
Although you can use other full featured GPS units, the bar bones hockey pucks have some benefits as far as I am concerned.
BE VERY CAREFUL about the Garmin 18/18X as there are a lot of USB Garmin 18/18X units, you NEED to make sure the unit you are going to buy is a SERIAL unit, not USB.
There was also some question is the Siren App would port the GPS info from the phone to the radio as well?? I recall a discussion on that, but do not recall 100% if the GPS info would be ported from the phone to the radio via the Siren App??