Do you mean a GPS tracker that can tell me where something is? Or do you just mean a GPS receiver that can tell me where I am? There's a huge difference.
For $5 one off cost per item and say $15 for the monthly service I'd stick them to pretty much everything.
NB Also depends on size and battery life of tracker and accuracy (e.g. finding a Kindle in a house requires a small tracker and reasonably good accuracy).