How cheap does it need to be? How accurate does it need to be?
You could get a "mini A8" tracker. This is a basic GSM cellular phone radio tied to a few other sensors and a little lipo battery
. This could be tucked away running on the trailers battery
. If the thieves disconnected the trailer battery
thinking they were smart, it would continue running a few days on the built in one.
This unit is only about $11.00 You will only need a gsm SIM card to pop in it for it to work. For the price, it doesn't have a GPS or A-GPS antenna. Hence why I asked how accurate you need it. What it does do is location based tracking calculated by triangulating the signals of cellular towers. It's cheap, it's effective, and it is still pretty darn accurate.
Another neat feature besides being able to text it to request map locations, is that if you call it, it will silently answer and let you hear through its built in mic. This could also give you vital clues as to who took the trailer. It will work perfectly with a pre-paid sim card. This means the yearly operating costs are nearly zero.
Here is an example link to one, but look around.
GPS Tracker Global Realtime 4BANDS Sim GSM GPRS Security Tracking Device Mini A8 | eBay
Now there are ones that are more like $40 or so if you really need hardcore GPS tracking down to a few meters of accuracy. Same deal, pre paid sim will work.