This is a tall order. Let me recap what I understand from the original question as well as comments made:
You have about 500 devices, each costing about US$200 each. You want to protect them from being stolen from an office park, and ideally there would be a tracking system so they could be recovered. The solution shouldn't increase the per-unit cost much, either in initial cost or recurring (monthly) costs.
Let me start my answer by telling you what won't work...
GPS: You didn't say what these devices are, but I am going to assume that they will be indoors almost all of the time. GPS reliability indoors is almost zero. If you are careful you could get one to work, but metal and concrete effectively block GPS signals. Wood does block the signal, but not as effectively as metal and concrete. The point is, GPS is not going to be the ultimate solution for your location requirements.
ZigBee, Bluetooth, Wireless: None of these are reliable enough to use for your communications. They would be OK within your office park, but not outside of it where you are essentially hoping that there is an unsecured network that you can use.
Pager/Cell-phone: From a technological point of view these would probably be your best solution, but the monthly costs will be prohibitive. As @kenny said, US$15/month would be the lowest price.
Battery Life: With any of these solutions you will be very limited by battery life. You might get a couple of days, max, and that assumes that you have enough space to put a reasonable size battery in it and the stuff required for 500 people to charge it.
So, here are some "out of the box" ideas that might work for you. If it doesn't work then maybe it will spark an idea or two.
Use WiFi, but not for tracking. When the unit powers up it tries to establish a Wifi connection with the office park network. If it can't then the unit simply shuts down. In this way, if the unit is taken out of the office park it just doesn't work. While this doesn't do any tracking/recovery it does reduce the motivation for theft. After boot, the WiFi module can be powered down thus saving battery life.
Use medium or long range RFID to detect the unit leaving the park. For example, place some RFID readers at the doors to buildings and log when a unit goes through. Combine this with security camera video and you can identify who took the unit. The huge advantage with this is that it requires no batteries and the cost per unit is very low. The initial investment might be high, but after that it doesn't really matter if your tracking 500 units or 5,000 units. And since it's RFID, you can protect just about anything without a huge engineering effort.
Hope that helps!
New info: Units will be mostly outside. Location tracking is most important within the office park/campus. Theft prevention isn't top priority.
In that case, I'd just use WiFi for communications with or without a GPS for location. Maybe include a GPS but use WiFi triangulation for when GPS reception isn't quite working (since you already have a WiFi module). If you find out that WiFi triangulation is good enough then you can skip the GPS on later units.
The main issue I see with this is the expense of the WiFi & GPS stuff. I'm estimating that it would cost about US$75-100 for such a circuit, which is not an insignificant fraction of the unit cost. I don't really see an alternative that would be as practical.
I still don't think ZigBee or Bluetooth is reasonable. It would work, but would require extra infrastructure. With WiFi, you can at least share the costs with the I.T. department.