Personally I don't think the cost is worth it. And I have GPS on a bunch of my equipment. Between the initial cost. Needing a battery and way to keep it charged and depending which model the service charge after a little while you have more money in making sure it doesn't get stolen then what the trailer is worth.
If it were me I would get a good hitch lock and a very heavy duty chain and chain the two axles and tires through the frame to make it harder.
Case Hardened chain & locks......pretty cheap insurance against trailer theft! I also re-registered my 16ft trailer as a regular utility trailer instead of a farm trailer and stamped the issued VIN multiple times on every part (axles, etc)and on sides, back, front, & tongue of trailer.
All good advice......Having managed an equipment fleet of over 150 pieces of equipment ranging from trailers to D-10 CATS; I can tell you that if thieves want to steal something- THEY WILL!
One infamous case I remember was having a D-8 CAT stolen off a jobsite in Dallas, It just VANISHED on labor day weekend 2011.
To catch crooks around the equipment yard I used the double bait method.... I would leave a fuel tank, trailer, four wheeler, ect. in a secluded area and put a broken game camera in plain sight as if it was there to catch someone stealing.
Then in a higher out of reach area that would require a ladder to access it; I would post a much smaller camera so that it could see both the equipment and the broken game camera.
The thieves come in and once they remove/steal the busted game camera they become more relaxed and usually you get a lot of good footage of them.